math.bitwise: factor M\ fixnum (bit-count) to a separate word so it can be made intrinsic

basis/math/bitwise/bitwise.factor

@@ -84,7 +84,7 @@ DEFER: byte-bit-count
 
 GENERIC: (bit-count) ( x -- n )
 
-M: fixnum (bit-count)
+: fixnum-bit-count ( x -- n )
     0 swap [
         dup 0 >
     ] [
@@ -92,6 +92,9 @@ M: fixnum (bit-count)
         [ + ] dip
     ] while drop ;
 
+M: fixnum (bit-count)
+    fixnum-bit-count ; inline
+
 M: bignum (bit-count)
     dup 0 = [ drop 0 ] [
         [ byte-bit-count ] [ -8 shift (bit-count) ] bi +