Make bytevectors accessible using the generalized-vector API.

Make bytevectors accessible using the generalized-vector API.

As a side effect, this allows compilation of literal bytevectors
("#vu8(...)"), which gets done by the generic array handling
of the GLIL->assembly compiler.

* doc/ref/api-compound.texi (Generalized Vectors): Mention bytevectors.
  (Arrays, Array Syntax): Likewise.

* doc/ref/api-data.texi (Bytevectors as Generalized Vectors): New node.

* libguile/bytevectors.c (scm_i_bytevector_generalized_set_x): New.

* libguile/bytevectors.h (scm_i_bytevector_generalized_set_x): New
  declaration.

* libguile/srfi-4.c (scm_i_generalized_vector_type,
  scm_array_handle_uniform_element_size,
  scm_array_handle_uniform_writable_elements): Add support for
  bytevectors.

* libguile/unif.c (type_creator_table): Add `vu8'.
  (bytevector_ref, bytevector_set): New functions.
  (memoize_ref, memoize_set): Add support for bytevectors.

* libguile/vectors.c (scm_is_generalized_vector,
  scm_c_generalized_vector_length, scm_c_generalized_vector_ref,
  scm_c_generalized_vector_set_x): Add support for bytevectors.

* test-suite/tests/bytevectors.test ("Generalized Vectors"): New test
  set.