* bitmap.c (bitmap_element_free, bitmap_element_link,
bitmap_elt_insert_after, bitmap_and, bitmap_and_compl,
bitmap_and_compl, bitmap_ior, bitmap_ior_into, bitmap_xor,
bitmap_xor_into): Added code to properly maintain the variants
associated with the CURRENT and HEAD fields.
(bitmap_popcount, bitmap_clear_range, bitmap_compl_and_into): New
functions. * bitmap.h: Added defs for bitmap_popcount,
bitmap_clear_range, and bitmap_compl_and_into.