board: use xor instead of masks where possible
succs: make functions static
ai: make functions static
--reverse: only reverse piece succs

It's the only thing that matters
negamax: add beta_orig
printBoard: remove boardEval call

Let's not couple board.c to ai.c
add opening book support

empty book though!
ai,book-gen: mark possibly unused variables
Makefile: silent bookgen
add --no-book option
add --seed option
book-gen: sort and remove dups

Do we really want this? In any case we'll disable it later
book: only randomize if shuffle is set
gitignore: add book-gen and book.gen
book-gen: prettier output
common: add clamp macro
stats: prettier output
ai: comments and assert's

The assert(0) is because for now we have no TT. Should be removed once
we do.
chess: add null move heuristic

Pretty basic I think, might be wrong... Shows clear (3x) improvements on
some boards, while not being measurably worse on other.
make_tests.sh: show each game lasting time