precompiles: Add EC mixed-add formula

[chfast]

Another -10% time reduction.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.50%. Comparing base (26cdd85) to head (142b394).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1170      +/-   ##
==========================================
+ Coverage   94.48%   94.50%   +0.01%     
==========================================
  Files         169      169              
  Lines       18014    18057      +43     
==========================================
+ Hits        17021    17064      +43     
  Misses        993      993              

Flag	Coverage Δ
eof_execution_spec_tests	20.84% <0.00%> (-0.05%)	⬇️
ethereum_tests	27.23% <95.74%> (+0.15%)	⬆️
ethereum_tests_silkpre	19.73% <95.65%> (+0.19%)	⬆️
execution_spec_tests	21.20% <0.00%> (-0.06%)	⬇️
unittests	91.68% <100.00%> (+0.01%)	⬆️

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Files with missing lines	Coverage Δ
lib/evmone_precompiles/bn254.cpp	100.00% <100.00%> (ø)
lib/evmone_precompiles/ecc.hpp	100.00% <100.00%> (ø)
lib/evmone_precompiles/secp256k1.cpp	100.00% <100.00%> (ø)

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