fix: determine correct bounds of custom types

[zrho]

Import now determines the bound of the custom type correctly.

Fixes #1876.

[hugrbot]

This PR contains breaking changes to the public Rust API.
Please deprecate the old API instead (if possible), or mark the PR with a ! to indicate a breaking change.

cargo-semver-checks summary

--- failure auto_trait_impl_removed: auto trait no longer implemented ---

Description:
A public type has stopped implementing one or more auto traits. This can break downstream code that depends on the traits being implemented.
      ref: https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.38.0/src/lints/auto_trait_impl_removed.ron

Failed in:
type DescendantsGraph is no longer Sync, in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-core/src/hugr/views/descendants.rs:28
type DescendantsGraph is no longer Sync, in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-core/src/hugr/views/descendants.rs:28

--- failure enum_variant_added: enum variant added on exhaustive enum ---

Description:
A publicly-visible enum without #[non_exhaustive] has a new variant.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#enum-variant-new
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.38.0/src/lints/enum_variant_added.ron

Failed in:
variant ImportError:ExtensionType in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-core/src/import.rs:57

--- failure auto_trait_impl_removed: auto trait no longer implemented ---

Description:
A public type has stopped implementing one or more auto traits. This can break downstream code that depends on the traits being implemented.
      ref: https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.38.0/src/lints/auto_trait_impl_removed.ron

Failed in:
type LLVMSumValue is no longer UnwindSafe, in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-llvm/src/sum.rs:181
type LLVMSumValue is no longer RefUnwindSafe, in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-llvm/src/sum.rs:181
type LLVMSumType is no longer UnwindSafe, in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-llvm/src/sum.rs:32
type LLVMSumType is no longer RefUnwindSafe, in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-llvm/src/sum.rs:32
type LLVMSumType is no longer UnwindSafe, in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-llvm/src/sum.rs:32
type LLVMSumType is no longer RefUnwindSafe, in /home/runner/work/hugr/hugr/PR_BRANCH/hugr-llvm/src/sum.rs:32

--- failure function_missing: pub fn removed or renamed ---

Description:
A publicly-visible function cannot be imported by its prior path. A `pub use` may have been removed, or the function itself may have been renamed or removed entirely.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.38.0/src/lints/function_missing.ron

Failed in:
function hugr_llvm::sum::elidable_type, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:24

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn is no longer available under its prior name. It may have been renamed or removed entirely.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.38.0/src/lints/inherent_method_missing.ron

Failed in:
LLVMSumValue::get_type, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:529
LLVMSumValue::tag_type, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:672
LLVMSumValue::num_variants, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:674
LLVMSumType::value_type, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:96
LLVMSumType::tag_type, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:98
LLVMSumType::num_variants, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:100
LLVMSumType::num_fields_for_variant, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:103
LLVMSumType::fields_for_variant, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:106
LLVMSumType::try_from_hugr_type, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:115
LLVMSumType::value, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:168
LLVMSumType::value_type, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:96
LLVMSumType::tag_type, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:98
LLVMSumType::num_variants, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:100
LLVMSumType::num_fields_for_variant, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:103
LLVMSumType::fields_for_variant, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:106
LLVMSumType::try_from_hugr_type, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:115
LLVMSumType::value, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-llvm/src/sum.rs:168

--- failure enum_missing: pub enum removed or renamed ---

Description:
A publicly-visible enum cannot be imported by its prior path. A `pub use` may have been removed, or the enum itself may have been renamed or removed entirely.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.38.0/src/lints/enum_missing.ron

Failed in:
enum hugr_passes::RemoveDeadFuncsError, previously in file /home/runner/work/hugr/hugr/BASELINE_BRANCH/hugr-passes/src/dead_funcs.rs:20

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 86.54%. Comparing base (b38a2e8) to head (5786561).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
hugr-core/src/import.rs	60.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1888      +/-   ##
==========================================
+ Coverage   86.52%   86.54%   +0.01%     
==========================================
  Files         194      195       +1     
  Lines       35250    35591     +341     
  Branches    32063    32404     +341     
==========================================
+ Hits        30501    30803     +302     
- Misses       2973     3014      +41     
+ Partials     1776     1774       -2     

Flag	Coverage Δ
python	92.34% <ø> (ø)
rust	85.97% <60.00%> (+0.02%)	⬆️

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[cqc-alec]

Checked that the test in https://github.com/CQCL/hugr/tree/doug/bounds-bug now does not panic. LGTM.