(GC_CHECK_STRING_BYTES): Temporarily define, for bug

(GC_CHECK_STRING_BYTES): Temporarily define, for bug
hunting.
(struct sdata) [GC_CHECK_STRING_BYTES]: Always record the string's
size in the sdata structure.
(SDATA_NBYTES, SDATA_DATA): New macros.
(SDATA_OF_STRING, SDATA_SIZE) [GC_CHECK_STRING_BYTES]: Define
differently for the different layout of the sdata structure.
(allocate_string_data) [GC_CHECK_STRING_BYTES]: Record string size
in sdata.
(sweep_strings, compact_small_strings) [GC_CHECK_STRING_BYTES]:
Check that size recorded in the string size and size recorded in
the sdata structure agree.