Use a TC7 tag instead of a SMOB for bytevectors.

Use a TC7 tag instead of a SMOB for bytevectors.

* libguile/bytevectors.c (scm_tc16_bytevector): Remove.
  (SCM_BYTEVECTOR_SET_LENGTH, SCM_BYTEVECTOR_SET_CONTENTS,
  SCM_BYTEVECTOR_SET_INLINE, SCM_BYTEVECTOR_SET_ELEMENT_TYPE,
  make_bytevector_from_buffer, scm_is_bytevector,
  scm_bootstrap_bytevectors): Adjust to the SMOB->tc7 change.
  (scm_i_print_bytevector): New, formerly `print_bytevector ()'.
  (bytevector_equal_p): Remove.

* libguile/bytevectors.h (SCM_BYTEVECTOR_LENGTH,
  SCM_BYTEVECTOR_CONTENTS, SCM_BYTEVECTOR_P): Adjust to SMOB->tc7
  change.
  (SCM_BYTEVECTOR_FLAGS, SCM_SET_BYTEVECTOR_FLAGS): New macros.
  (scm_tc16_bytevector): Remove declaration.
  (scm_i_print_bytevector): New declaration.

* libguile/eq.c (scm_equal_p): Handle `scm_tc7_bytevector'.

* libguile/evalext.c (scm_self_evaluating_p): Likewise.

* libguile/print.c (iprin1): Likewise.

* libguile/tags.h (scm_tc7_bytevector): New.
  (scm_tc7_unused_8): Remove.

* libguile/validate.h (SCM_VALIDATE_BYTEVECTOR): Adjust.

* test-suite/tests/bytevectors.test ("Datum
  Syntax")["self-evaluating?"]: New test.