add registry of vector constructors, make-generalized-vector

add registry of vector constructors, make-generalized-vector

* libguile/generalized-vectors.h:
* libguile/generalized-vectors.c: Add a registry of vector constructors.
  (scm_make_generalized_vector): New public function, constructs a
  vector of a given type.

* libguile/bitvectors.c:
* libguile/bytevectors.c:
* libguile/srfi-4.c:
* libguile/strings.c:
* libguile/vectors.c: Register vector constructors.

* libguile/extensions.c (scm_init_extensions): No need to NULL the list
  of registered extensions here, the static init does it for us. Allows
  scm_c_register_extension to be called before scm_init_extensions.

* libguile/init.c (scm_i_init_guile): Move array initialization earlier,
  so e.g. scm_init_strings has access to a valid list of array element
  types when registering its vector constructor.