Rewrite unstoppable pawns evaluation

Instead of current code, give a bonus according to the frontmost
square among candidate + passed pawns.

This is a big simplification that removes a lot of accurate code
substituting it with a statistically based one using the common
'bonus' scheme, leaving to the search to sort out the details.

Results are equivalent but code is much less and, as an added bonus,
we now store candidates bitboard in pawns hash and allow this
info to be used in evaluation. This paves the way to possible
candidate pawns evaluations together with all the other pieces,
as we do for passed.

Patch passed short TC
LLR: 2.96 (-2.94,2.94) [-1.50,4.50]
Total: 16927 W: 3462 L: 3308 D: 10157

Then failed (quite quickly) at long TC
LLR: -2.95 (-2.94,2.94) [0.00,6.00]
Total: 8451 W: 1386 L: 1448 D: 5617

But when ran with a conclusive 40K fixed games at 60 secs it proved
almost equivalent to original one.

ELO: 1.08 +-2.0 (95%) LOS: 85.8%
Total: 40000 W: 6739 L: 6615 D: 26646

bench: 3884003

src/pawns.h

@@ -37,6 +37,7 @@ struct Entry {
   Score pawns_value() const { return value; }
   Bitboard pawn_attacks(Color c) const { return pawnAttacks[c]; }
   Bitboard passed_pawns(Color c) const { return passedPawns[c]; }
+  Bitboard candidate_pawns(Color c) const { return candidatePawns[c]; }
   int pawns_on_same_color_squares(Color c, Square s) const { return pawnsOnSquares[c][!!(DarkSquares & s)]; }
   int semiopen(Color c, File f) const { return semiopenFiles[c] & (1 << int(f)); }
   int semiopen_on_side(Color c, File f, bool left) const {
@@ -59,6 +60,7 @@ struct Entry {
 
   Key key;
   Bitboard passedPawns[COLOR_NB];
+  Bitboard candidatePawns[COLOR_NB];
   Bitboard pawnAttacks[COLOR_NB];
   Square kingSquares[COLOR_NB];
   int minKPdistance[COLOR_NB];