feat[venom]: add calloca instruction

[charles-cooper]

What I did

promote the callee buffer (allocated at call site) to an alloca-like instruction.

there are a couple minor regressions in here:
one is fixed in #4419
the other is filed in #4420.

How I did it

How to verify it

Commit message

this commit promotes the callee buffer (allocated at call site) to an
alloca-like instruction, `calloca`. this allows us to treat all buffers
generated by the frontend abstractly, instead of needing to do memory
analysis.

`calloca` additionally has a parameter to help identify which call-site
it is associated with, as a hint for alias and reordering analyses.

Description for the changelog

Cute Animal Picture

[codecov]

Codecov Report

Attention: Patch coverage is 94.59459% with 2 lines in your changes missing coverage. Please review.

Project coverage is 92.02%. Comparing base (c208b95) to head (319ddd4).

Files with missing lines	Patch %	Lines
vyper/venom/venom_to_assembly.py	0.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4376      +/-   ##
==========================================
+ Coverage   92.01%   92.02%   +0.01%     
==========================================
  Files         119      119              
  Lines       16692    16724      +32     
  Branches     2805     2810       +5     
==========================================
+ Hits        15359    15391      +32     
  Misses        915      915              
  Partials      418      418              

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