Also give bytevectors a private type (a struct).
* libguile/bytevectors.h (SCM_BYTEVECTOR_HEADER_SIZE): Remove.
(SCM_BYTEVECTOR_LENGTH):
(SCM_BYTEVECTOR_CONTENTS): Proxy to the C accessors.
(SCM_BYTEVECTOR_PARENT): Remove from public API.
(SCM_BYTEVECTOR_P, SCM_VALIDATE_BYTEVECTOR): Make public.
(scm_c_bytevector_contents): New function.
* libguile/bytevectors-internal.h: New file.
* libguile/Makefile.am (noinst_HEADERS): Add new file.
* libguile/bytevectors.c:
* libguile/array-handle.c:
* libguile/arrays.c:
* libguile/foreign.c:
* libguile/goops.c:
* libguile/init.c:
* libguile/loader.c:
* libguile/print.c:
* libguile/r6rs-ports.c:
* libguile/srfi-4.c:
* libguile/strings.c: Adapt to use bytevectors-internal.h as needed, and
sometimes to use the internal bytevector type.
