* alloc.c: Remove unportable assumptions about struct layout.

* alloc.c: Remove unportable assumptions about struct layout.

(SDATA_SELECTOR, SDATA_DATA_OFFSET): New macros.
(SDATA_OF_STRING, SDATA_SIZE, allocate_string_data):
(allocate_vectorlike, make_pure_vector): Use the new macros,
plus offsetof, to remove unportable assumptions about struct layout.
These assumptions hold on all porting targets that I know of, but
they are not guaranteed, they're easy to remove, and removing them
makes further changes easier.