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.
Correctly score enpassant captures

Surprisingly this rare case was not considered
when scoring a capture.

Also take in account that in the promotion case
we gain a new piece (typically a queen) but we
lose the promoting pawn.

These small issues were present since Glaurung times!

Found while browsing DiscoCheck sources

bench: 5400063
Rename prevents_move() to refutes()

Better! From DiscoCheck.

No functional change.
Templetize score_xxx() functions

So to be style-wise aligned with the corresponding
generate() functions.

No functional change.
Be clear about not LMR the ttMove

Currently a ttMove is reduced with ss->reduction = DEPTH_ZERO,
so it is actually not reduced (as it should be), but the
trick works just becuase it happens that ttMove is the first
to be tried and

reduction(depth, 1)

Always returns zero. So explicitly forbid reduction of ttMove
in the LMR condition. This is much clear and self-documented.

No functional change.
Some renaming in split()

Naming suggested by jundery.

No functional change.