Retire the misdesigned StepAttacks[] array.

StepAttacks[] is misdesigned, the color dependance is specific
to pawns, and trying to generalise to king and knights, proves
neither useful nor convinient in practice.

So this patch reformats the code with the following changes:

- Use PieceType instead of Piece in attacks_() functions

- Use PseudoAttacks for KING and KNIGHT

- Rename StepAttacks[] into PawnAttacks[]

Original patch and idea from Alain Savard.

No functional change.

Closes #1086

src/movegen.cpp

@@ -80,7 +80,7 @@ namespace {
 
     // Knight promotion is the only promotion that can give a direct check
     // that's not already included in the queen promotion.
-    if (Type == QUIET_CHECKS && (StepAttacksBB[W_KNIGHT][to] & ksq))
+    if (Type == QUIET_CHECKS && (PseudoAttacks[KNIGHT][to] & ksq))
         *moveList++ = make<PROMOTION>(to - D, to, KNIGHT);
     else
         (void)ksq; // Silence a warning under MSVC
@@ -346,7 +346,7 @@ ExtMove* generate<QUIET_CHECKS>(const Position& pos, ExtMove* moveList) {
      if (pt == PAWN)
          continue; // Will be generated together with direct checks
 
-     Bitboard b = pos.attacks_from(Piece(pt), from) & ~pos.pieces();
+     Bitboard b = pos.attacks_from(pt, from) & ~pos.pieces();
 
      if (pt == KING)
          b &= ~PseudoAttacks[QUEEN][pos.square<KING>(~us)];