Optimization for multiple memory-zeroing instructions

[iamdefinitelyahuman]

What I did

Add an optimization for zeroing multiple memory slots at once.

Closes #2144

How I did it

When the LLL contains multiple sequential instructions to MSTORE n 0 or CALLDATACOPY m CALLDATALSIZE n, and the locations being zero'd within memory are sequential, merge these instructions into a single zeroing via calldatacopy.

How to verify it

Run tests.

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[codecov-commenter]

Codecov Report

Merging #2174 into master will increase coverage by 0.04%.
The diff coverage is 96.66%.

@@            Coverage Diff             @@
##           master    #2174      +/-   ##
==========================================
+ Coverage   85.25%   85.29%   +0.04%     
==========================================
  Files          83       83              
  Lines        8367     8397      +30     
  Branches     2019     2029      +10     
==========================================
+ Hits         7133     7162      +29     
  Misses        732      732              
- Partials      502      503       +1     

Impacted Files	Coverage Δ
vyper/optimizer.py	83.33% <96.66%> (+3.92%)	⬆️

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