fix: allow enum as mapping key

[tserg]

What I did

Fix #3247.

How I did it

Set _as_hashmap_key = True for EnumT.

How to verify it

See test.

Commit message

fix: allow enum as mapping key

Description for the changelog

Allow enum as mapping key

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[codecov-commenter]

Codecov Report

Merging #3256 (9313549) into master (02339df) will decrease coverage by 0.31%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #3256      +/-   ##
==========================================
- Coverage   88.88%   88.57%   -0.31%     
==========================================
  Files          84       84              
  Lines       10598    10595       -3     
  Branches     2212     2144      -68     
==========================================
- Hits         9420     9385      -35     
- Misses        769      801      +32     
  Partials      409      409              

Impacted Files	Coverage Δ
vyper/semantics/types/user.py	85.62% <100.00%> (+0.04%)	⬆️
vyper/codegen/arithmetic.py	75.00% <0.00%> (-9.72%)	⬇️
vyper/ir/s_expressions.py	91.17% <0.00%> (-2.95%)	⬇️
vyper/builtins/functions.py	89.20% <0.00%> (-1.23%)	⬇️
vyper/ast/nodes.py	93.63% <0.00%> (-0.16%)	⬇️
vyper/codegen/abi_encoder.py	88.78% <0.00%> (-0.11%)	⬇️
vyper/semantics/analysis/module.py	86.15% <0.00%> (-0.08%)	⬇️
vyper/codegen/core.py	86.67% <0.00%> (-0.03%)	⬇️

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