Reimplement newBitboard to avoid string operations. (#75)

Instead of building a string of "0"s and "1"s and then converting it,
newBitboard now uses bit arithmetic to convert a map to a uint64. This
avoids memory allocation and a bunch of string operations.

bitboard.go

@@ -10,18 +10,13 @@ import (
 type bitboard uint64
 
 func newBitboard(m map[Square]bool) bitboard {
-	s := ""
+	var bb uint64
 	for sq := 0; sq < numOfSquaresInBoard; sq++ {
+		bb <<= 1
 		if m[Square(sq)] {
-			s += "1"
-		} else {
-			s += "0"
+			bb |= 1
 		}
 	}
-	bb, err := strconv.ParseUint(s, 2, 64)
-	if err != nil {
-		panic(err)
-	}
 	return bitboard(bb)
 }