feat[test]: add hevm harness for venom passes

[charles-cooper]

What I did

How I did it

How to verify it

Commit message

this commit adds `hevm` as a harness to some venom tests. to accomplish
this, it adds a new venom instruction "sink", which in the new
`hevm_check_venom` function gets compiled to a `return` instruction
so that hevm can check the returndata for equivalence. this allows us
to rely on bytecode equivalence of the compiled code to "prove" our
optimizations are sound.

the new pattern has been applied to the tests in
`test_algebraic_binopt.py` as a first pass, but it can (and should) be
applied to more venom tests in the future. and `hevm=False` kwarg has
been added for some cases where hevm cannot resolve the equivalence.

an hevm harness could also be added to vyper sources, but that is out
of scope for this PR and left for future work.

references:
- https://github.com/ethereum/hevm

Description for the changelog

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[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.09%. Comparing base (1aaa78e) to head (bed64d6).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
vyper/venom/venom_to_assembly.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4460      +/-   ##
==========================================
- Coverage   92.11%   92.09%   -0.02%     
==========================================
  Files         119      119              
  Lines       16933    16936       +3     
  Branches     2865     2866       +1     
==========================================
+ Hits        15597    15598       +1     
- Misses        917      919       +2     
  Partials      419      419              

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[HodanPlodky]

Do you want to leave this here or could be revert this?

[charles-cooper]

I'll leave it for people to review then remove

[HodanPlodky]

you could remove this one right (num_return_values)?

[charles-cooper]

Yea, looks like it