Allocate XPV* memory using sizeof() and the offset of the first real member.

Allocate XPV* memory using sizeof() and the offset of the first real member.

This avoids tripping over alignment differences between X* and x*_allocated,
because x*_allocated doesn't have a double in it.

Tidy up the documentation in comments. Eliminate relative_STRUCT_OFFSET() as
it is no longer used. Eliminate use of xpv_allocated, xpviv_allocated,
xpvav_allocated, xpvcv_allocated and xpvfm_allocated, but leave them in the
headers as they are publicly visible.