Remove some struct layout assumptions in bool vectors.

Remove some struct layout assumptions in bool vectors.

* alloc.c (bool_header_size): New constant.
(header_size, word_size): Move earlier, as they're now used earlier.
Use 'word_size' in a few more places, where it's appropriate.
(Fmake_bool_vector, sweep_vectors): Don't assume that there is no
padding before the data member of a bool vector.
(sweep_vectors): Use PSEUDOVECTOR_TYPEP, in an eassert, rather
than doing the check by hand with an abort ().