bytevectors have internal parent field

bytevectors have internal parent field

* libguile/bytevectors.h (SCM_BYTEVECTOR_HEADER_SIZE): Bump, giving
  bytevectors another word: a parent pointer.  Will allow for
  sub-bytevectors and efficient mmap bindings.

* libguile/bytevectors.c (make_bytevector):
  (make_bytevector_from_buffer): Init parent to #f.
  (scm_c_take_bytevector, scm_c_take_typed_bytevector): Another
  argument, the parent, which gets set in the bytevector.

* libguile/foreign.c (scm_pointer_to_bytevector): Use the parent field
  instead of registering a weak reference from bytevector to foreign
  pointer.

* libguile/objcodes.c (scm_objcode_to_bytecode): Use the parent field to
  avoid copying the objcode.

* libguile/srfi-4.c (DEFINE_SRFI_4_C_FUNCS):
* libguile/strings.c (scm_from_stringn):
* libguile/vm.c (really_make_boot_program):
* libguile/r6rs-ports.c (scm_get_bytevector_some)
  (scm_get_bytevector_all, bytevector_output_port_procedure): Set the
  parent to #f.