Allocate vectors in a contiguous memory area.
* libguile/vectors.c (scm_c_make_vector): Allocate the whole vector and
header with `scm_gc_malloc ()'.
(scm_vector_copy): Use `scm_c_make_vector ()'.
(scm_i_vector_free, MAKE_WEAK_VECTOR): Remove.
(allocate_weak_vector): Rename to...
(make_weak_vector): ... this. Change to return the whole weak vector,
allocated with `scm_gc_malloc_pointerless ()'.
(scm_i_make_weak_vector, scm_i_make_weak_vector_from_list): Use
`make_weak_vector ()'.
* libguile/vectors.h (SCM_I_VECTOR_HEADER_SIZE): New macro.
(SCM_I_VECTOR_ELTS): Write in terms of `SCM_I_VECTOR_WELTS ()'.
(SCM_I_VECTOR_WELTS): Update to the new representation.
(SCM_I_WVECT_EXTRA, SCM_I_SET_WVECT_EXTRA): Likewise.
(SCM_I_WVECT_GC_CHAIN, SCM_I_SET_WVECT_GC_CHAIN): Remove.
* libguile/weaks.h (SCM_I_WVECT_DELTA, SCM_I_SET_WVECT_DELTA): Remove.