Name functions along corresponding UCI commands

No functional change.
Simplify hidden_checkers()

De-templetize and pass color as function argument.
No speed change.

No functional change.
Retire ThisAndAdjacentFilesBB[]

It is unused. Also renamed attack_span_mask to
pawn_attack_span

No functional change.
Retire in_front_bb(Color c, Square s) overload

Explciitly call rank_of() in the few places where
it is used.

No functional change.
Don't explicitize enum values when not needed

Compiler will chose the correct values in sequential
order for you.

Also move file and rank bitboards definitions to
bitboard.h

No functional change.
Move SquareDistance[] to bitboard.cpp

No functional change.
Fix some stale comments

No functional change.
Use calloc() in TranspositionTable::set_size()

Function calloc() already initializes memory to
zero, so avoid calling clear() afterwards.

Also some renaming while there (inspired by DiscoCheck).

No functional change.