(enum_bitset_ops, enum_bitset_type): New types.

All instances of parameters of type enum bitset_opts are now of
type enum_bitset_opts, to conform to the C Standard, and similarly
for enum_bitset_type.

(bitset): Now points to a union, not to a struct.
All instances of "struct bitset_struct *" replaced with "bitset".

(bitset_and_or_, bitset_andn_or_, bitset_or_and_): New decls.