Simplify a condition in gives_check()

Now that aligned() is quite fast we can skip
some logic.

No functional change.
Rename CASTLE to CASTLING

It is call 'castling move', not 'castle move'

Noticed while reading DiscoCheck sources.

No functional change.
Assorted spelling fixes

No functional change.
Big assorted spelling fixes

No functional change.
Another round of spelling fixes

And also renamed a loop variable while there.

No functional change.
Re-fix a comment

No functional change.
Small improvment to Position::fen()

No functional change.
Remove redundant argument from hidden_checkers()

No functional change.
Shrink Position::is_draw()

No functional change.
(fix merge conflict before a new try of "Big assorted spelling fixes")

No functional change. by Richard Lloyd