perf!: Manual impl of Region/FlatRegion for improved perf

[aborgna-q]

Followup to #157. Avoids using FilteredGraph for the implementation of Region and FlatRegion and instead implements the graph traits manually to avoid doing full-graph traversals when we can do better/faster checks.

Closes #159. Closes #145 (BFS rather than DFS). Closes #135.

BREAKING CHANGE: Region and FlatRegion are no longer aliases of FilteredGraph

[hugrbot]

This PR contains breaking changes to the public Rust API.

cargo-semver-checks summary

--- 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/v0.36.0/src/lints/inherent_method_missing.ron

Failed in:
FilteredGraph::new_region, previously in file /home/runner/work/portgraph/portgraph/BASELINE_BRANCH/src/view/region.rs:26
FilteredGraph::new_flat_region, previously in file /home/runner/work/portgraph/portgraph/BASELINE_BRANCH/src/view/region.rs:105
FilteredGraph::new_region, previously in file /home/runner/work/portgraph/portgraph/BASELINE_BRANCH/src/view/region.rs:26
FilteredGraph::new_flat_region, previously in file /home/runner/work/portgraph/portgraph/BASELINE_BRANCH/src/view/region.rs:105

--- failure module_missing: pub module removed or renamed ---

Description:
A publicly-visible module cannot be imported by its prior path. A `pub use` may have been removed, or the module may have been renamed, removed, or made non-public.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.36.0/src/lints/module_missing.ron

Failed in:
mod portgraph::view::region, previously in file /home/runner/work/portgraph/portgraph/BASELINE_BRANCH/src/view/region.rs:1
mod portgraph::view::subgraph, previously in file /home/runner/work/portgraph/portgraph/BASELINE_BRANCH/src/view/subgraph.rs:1

--- failure struct_missing: pub struct removed or renamed ---

Description:
A publicly-visible struct cannot be imported by its prior path. A `pub use` may have been removed, or the struct 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/v0.36.0/src/lints/struct_missing.ron

Failed in:
struct portgraph::view::subgraph::Subgraph, previously in file /home/runner/work/portgraph/portgraph/BASELINE_BRANCH/src/view/subgraph.rs:51
struct portgraph::view::region::RegionContext, previously in file /home/runner/work/portgraph/portgraph/BASELINE_BRANCH/src/view/region.rs:35

[codecov]

Codecov Report

Attention: Patch coverage is 96.10136% with 20 lines in your changes missing coverage. Please review.

Project coverage is 82.29%. Comparing base (c2cd701) to head (b7a6e8a).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/view/flat_region.rs	96.58%	2 Missing and 6 partials ⚠️
src/hierarchy.rs	79.31%	6 Missing ⚠️
src/view/region.rs	97.60%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #162      +/-   ##
==========================================
+ Coverage   82.15%   82.29%   +0.14%     
==========================================
  Files          22       23       +1     
  Lines        5386     5830     +444     
  Branches     5386     5830     +444     
==========================================
+ Hits         4425     4798     +373     
- Misses        888      956      +68     
- Partials       73       76       +3     

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[aborgna-q]

I'll add some tests before merging