Remove piece lists

This patch removes the incrementally updated piece lists from the Position object.

This has been tried before but always failed. My reasons for trying again are:

* 32-bit systems (including phones) are now much less important than they were some years ago (and are absent from fishtest);
* NNUE may have made SF less finely tuned to the order in which moves were generated.

STC:
LLR: 2.94 (-2.94,2.94) {-1.25,0.25}
Total: 55272 W: 5260 L: 5216 D: 44796
Ptnml(0-2): 208, 4147, 18898, 4159, 224
https://tests.stockfishchess.org/tests/view/5fc2986a42a050a89f02c926

LTC:
LLR: 2.96 (-2.94,2.94) {-0.75,0.25}
Total: 16600 W: 673 L: 608 D: 15319
Ptnml(0-2): 14, 533, 7138, 604, 11
https://tests.stockfishchess.org/tests/view/5fc2f98342a050a89f02c95c

closes https://github.com/official-stockfish/Stockfish/pull/3247

Bench: 3940967

src/pawns.cpp

@@ -91,7 +91,7 @@ namespace {
     Square s;
     bool backward, passed, doubled;
     Score score = SCORE_ZERO;
-    const Square* pl = pos.squares<PAWN>(Us);
+    Bitboard b = pos.pieces(Us, PAWN);
 
     Bitboard ourPawns   = pos.pieces(  Us, PAWN);
     Bitboard theirPawns = pos.pieces(Them, PAWN);
@@ -104,8 +104,9 @@ namespace {
     e->blockedCount += popcount(shift<Up>(ourPawns) & (theirPawns | doubleAttackThem));
 
     // Loop through all pawns of the current color and score each pawn
-    while ((s = *pl++) != SQ_NONE)
-    {
+    while (b) {
+        s = pop_lsb(&b);
+
         assert(pos.piece_on(s) == make_piece(Us, PAWN));
 
         Rank r = relative_rank(Us, s);