bytevectors: Add a C-friendly API.

bytevectors: Add a C-friendly API.

* doc/ref/api-data.texi (Bytevector Manipulation): Add
  `scm_is_bytevector ()', `scm_c_bytevector_length ()',
  `scm_c_bytevector_length ()', and `scm_c_bytevector_set_x ()'.

* libguile/bytevectors.c (scm_is_bytevector, scm_c_bytevector_length,
  scm_c_bytevector_ref, scm_c_bytevector_set_x): New functions.
  (scm_bytevector_p): Use `scm_is_bytevector ()'.
  (scm_bytevector_length): Use `scm_c_bytevector_length ()'.

* libguile/bytevectors.h (scm_is_bytevector, scm_c_bytevector_length,
  scm_c_bytevector_ref, scm_c_bytevector_set_x): New declarations.