Fix another case of equal_computations() with NoneConst and add a test.
Test the previous PR fix of equal_computations()
Fix rare compilation crash
Hide callbacks detail if they are fast.
remove useless clone
small code refactoring
code simplification
Add assert, remove useless check and move variable closer to where they are used.
move costly check later
Make cheap comparison earlier. This make an scan opt from 4s to 1.Xs.
Add some doc how to test multi-core implementation.
more line not used.
Merge pull request #1931 from nouiz/eq_computation

Crash fix equal_computations() again and scan opt speed up.