transient storage keyword

[charles-cooper]

What I did

How I did it

todo:
- [ ] tests

• update nonreentrancy to use tload/tstore

- [ ] separate allocator for transient(?)

test cases will come in a follow up once we get the ability to test the functionality via py-evm.

How to verify it

Commit message

    experimentally add support for transient storage via a new `transient`
    keyword, which works like `immutable` or `constant`, ex.:

    ```vyper
    my_transient_variable: transient(uint256)
    ```

    this feature is considered experimental until py-evm adds support
    (giving us the ability to actually test it). so this commit leaves the
    default evm version as "shanghai" for now. it blocks the feature on
    pre-cancun EVM versions, so users can't use it by accident - the only
    way to use it is to explicitly enable it via `--evm-version=cancun`.

Description for the changelog

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[codecov-commenter]

Codecov Report

Merging #3373 (8bebbb3) into master (27b6b89) will decrease coverage by 0.04%.
The diff coverage is 83.33%.

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@@            Coverage Diff             @@
##           master    #3373      +/-   ##
==========================================
- Coverage   89.32%   89.29%   -0.04%     
==========================================
  Files          84       84              
  Lines       10777    10789      +12     
  Branches     2459     2461       +2     
==========================================
+ Hits         9627     9634       +7     
- Misses        754      757       +3     
- Partials      396      398       +2     

Impacted Files	Coverage Δ
vyper/cli/vyper_compile.py	68.38% <ø> (ø)
vyper/evm/opcodes.py	100.00% <ø> (ø)
vyper/semantics/data_locations.py	100.00% <ø> (ø)
vyper/semantics/namespace.py	97.29% <ø> (ø)
vyper/codegen/core.py	86.74% <50.00%> (ø)
vyper/codegen/function_definitions/utils.py	66.66% <66.66%> (-6.67%)	⬇️
vyper/ast/nodes.py	93.74% <100.00%> (-0.14%)	⬇️
vyper/codegen/context.py	92.74% <100.00%> (+0.05%)	⬆️
vyper/codegen/expr.py	87.70% <100.00%> (+0.02%)	⬆️
vyper/evm/address_space.py	100.00% <100.00%> (ø)
... and 2 more

... and 1 file with indirect coverage changes

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