refactor: ecadd() and ecmul() codegen

[charles-cooper]

slight cleanup: refactor ecadd and ecmul to use make_setter and share code, so we don't need to do pointer arithmetic

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[codecov-commenter]

Codecov Report

Merging #3587 (b378478) into master (78fa8dd) will decrease coverage by 0.33%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #3587      +/-   ##
==========================================
- Coverage   89.10%   88.77%   -0.33%     
==========================================
  Files          85       85              
  Lines       11390    11385       -5     
  Branches     2586     2585       -1     
==========================================
- Hits        10149    10107      -42     
- Misses        817      852      +35     
- Partials      424      426       +2     

Files Changed	Coverage Δ
vyper/builtins/functions.py	90.52% <100.00%> (-0.04%)	⬇️

... and 5 files with indirect coverage changes

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