precompiles: Fallback to single mul in BLS MSM

[chfast]

If the number of valid and non-zero inputs to BLS MSM is 1 use simpler single point multiplication.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.21%. Comparing base (8e4a055) to head (01e0cfb).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1046   +/-   ##
=======================================
  Coverage   94.21%   94.21%           
=======================================
  Files         153      153           
  Lines       15934    15940    +6     
=======================================
+ Hits        15012    15018    +6     
  Misses        922      922           

Flag	Coverage Δ
eof_execution_spec_tests	17.53% <0.00%> (-0.01%)	:arrow_down:
ethereum_tests	27.33% <0.00%> (-0.02%)	:arrow_down:
ethereum_tests_silkpre	19.14% <0.00%> (-0.02%)	:arrow_down:
execution_spec_tests	20.48% <100.00%> (+0.03%)	:arrow_up:
unittests	89.01% <100.00%> (+<0.01%)	:arrow_up:

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Files with missing lines	Coverage Δ
lib/evmone_precompiles/bls.cpp	100.00% <100.00%> (ø)

[chfast]

Maybe this is sometimes faster, but not needed for correctness, so let's skip it.