Passed pawn eval

Use a stepped function to evaluate bonuses and add the bonus to the
middle game

bench: 4857939
Avoid explicit bitwise operators

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
Merge 'passed_pawns' tweaks

Good at both short and long TC

15+0.05
LLR: 2.96 (-2.94,2.94)
Total: 28220 W: 5531 L: 5349 D: 17340

TC 60+0.05
LLR: 2.95 (-2.94,2.94)
Total: 12612 W: 2221 L: 2057 D: 8334

bench: 4857939
Simplify previous patch

No functional change.
Some code reformat in evaluate_pieces

No functional change.
Use Them instead of ~Us

Unortunatly we have no guarantee that the call to
operator~(Color c) is resolved at compile time.

Perhaps the solution would be to use C++11 const_expr,
but for now simply use the good old-style ternary operator
that works as expected.

No functional change.
Merge pull request #2 from mcostalba/master

update branch