Switch bitset.h/raw_bitset.h to size_t instead of unsigned size parameters.

Switch bitset.h/raw_bitset.h to size_t instead of unsigned size parameters.

Note that this is an ugly change because ALL index variables used inside
bitset_foreach() MUST be switched to size_t now, or the magical

elm != (size_t) -1;

will fail :-(

[r28199]