feat: bitwise not and xor for enums

[charles-cooper]

What I did

How I did it

How to verify it

see tests

Commit message

add bitwise not and bitwise xor ops for enums. note that the
implementation uses xors with a special mask to avoid flipping upper
bits that do not belong to the enum (ex. using the `not` opcode on an
enum with 5 members does not make sense, because the 6th and higher bits
would get flipped to 1, which is out of bounds for the enum).

also, allow 0 as a valid enum value (this makes sense because storage
slots are 0 by default, and we have to assume that any storage slot with
type `MyEnum` is a valid enum value. in other words, `empty(Type)` is a
member of `Type` for any `Type`).

lastly, fix codegen for `not in` enum - it was being incorrectly handled
the same as `in`.

Description for the changelog

add bitwise not and xor operators for enums.

Cute Animal Picture

[codecov-commenter]

Codecov Report

Merging #2925 (b2c0575) into master (1e5770c) will decrease coverage by 0.11%.
The diff coverage is 67.34%.

@@            Coverage Diff             @@
##           master    #2925      +/-   ##
==========================================
- Coverage   88.34%   88.22%   -0.12%     
==========================================
  Files          97       97              
  Lines       10739    10772      +33     
  Branches     2555     2565      +10     
==========================================
+ Hits         9487     9504      +17     
- Misses        797      809      +12     
- Partials      455      459       +4     

Impacted Files	Coverage Δ
vyper/ir/optimizer.py	95.01% <ø> (ø)
vyper/builtin_functions/functions.py	91.14% <35.71%> (-1.27%)	⬇️
vyper/semantics/types/user/enum.py	80.30% <50.00%> (+0.30%)	⬆️
vyper/codegen/expr.py	84.36% <76.47%> (-0.17%)	⬇️
vyper/ast/nodes.py	93.43% <85.71%> (-0.23%)	⬇️
vyper/codegen/core.py	84.87% <100.00%> (-0.06%)	⬇️
vyper/semantics/types/value/numeric.py	85.05% <0.00%> (+4.59%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1e5770c...b2c0575. Read the comment docs.

[lgtm-com]

This pull request introduces 1 alert when merging 3037562 into 9a363dc - view on LGTM.com

new alerts:

• 1 for Variable defined multiple times