feat[venom]: add varname freshener

[charles-cooper]

for when the varnames need to be "freshened" up. can be helpful for debugging

What I did

How I did it

How to verify it

Commit message

add a "freshen_varnames" method to IRFunction, which can help debugging
by renaming all variables, restarting from 0.

Description for the changelog

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https://www.youtube.com/watch?v=-JfEJq56IwI&t=58s

[codecov]

Codecov Report

Attention: Patch coverage is 16.66667% with 10 lines in your changes missing coverage. Please review.

Project coverage is 92.04%. Comparing base (18567f6) to head (dde121d).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
vyper/venom/function.py	16.66%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4484      +/-   ##
==========================================
- Coverage   92.09%   92.04%   -0.06%     
==========================================
  Files         120      120              
  Lines       17349    17361      +12     
  Branches     2935     2940       +5     
==========================================
+ Hits        15978    15980       +2     
- Misses        954      964      +10     
  Partials      417      417              

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