Retire history.h

And move the contents to movepick.cpp, where they are
mostly used.

Idea from DiscoCheck.

No functional change (bench 5379503)
Unify History and Gains under a single Stats class

Handling of History and Gains is almost the same, with
the exception of the update logic, so unify both
classes under a single Stats struct.

No functional change.
Merge remote-tracking branch 'upstream/master'