Clarify definition of capture_or_promotion()

No functional change.
Further simplify Bitboards init()

No functional change.
More readable init of MS1BTable[]

Because now it uses lsb(), the BSFTable[] must be
already initialized.

No functional change.
More work on Bitboards::init()

No functional change.
Merge branch 'master' into search
Increase child node futility pruning threshold

bench: 8039738