Replace hardcoded 128 by constant

No functional change.
Bonus for rook behind a passed

If our rook is behind a passed pawn, all
squares are defended.

One of the longest tests to pass !

Passed both short TC
LLR: 2.97 (-2.94,2.94)
Total: 44560 W: 9518 L: 9281 D: 25761

And long TC
LLR: 2.96 (-2.94,2.94)
Total: 61348 W: 11618 L: 11192 D: 38538

bench: 3787694
Enable LMR for dangerous moves

Passed both short TC
LLR: 2.96 (-2.94,2.94)
Total: 5598 W: 1250 L: 1125 D: 3223

And long TC
LLR: 2.97 (-2.94,2.94)
Total: 16441 W: 3102 L: 2912 D: 10427

bench: 4620975
Use frontmost_sq() and backmost_sq helpers

Should easier to read than the lsb() / msb() low
level functions.

No functional change.
Fix a bogus assert in allows()

Becuase castle is coded as "king captures the rook"
the to_sq(move), A1/8 or H1/8 is empty after the move,
leading to assert assert(p != NO_PIECE) in color_of().

Teach allows() asserts about castle and fix the crash.

Bug reported by Ryan Takker and tracked down by Tom Vijlbrief.

No functional change.