Allocate data for structures on an eight-byte boundary, as

Allocate data for structures on an eight-byte boundary, as
required by the tagging system.
* struct.c (alloc_struct): New function.
(scm_make_struct, scm_make_vtable_vtable): Call it.
* struct.h (scm_struct_n_extra_words): Bump to 3.
(scm_struct_i_ptr): New "field".
* gc.c (scm_gc_sweep): When we need to free the data, use the
information stored by alloc_struct to find the beginning of the
block allocated to the structure, so we can free it.