Suggested by Neil Jerram.

* libguile/foreign.h (SCM_FOREIGN_TYPE, SCM_FOREIGN_VALUE_REF,
  SCM_FOREIGN_VALUE_SET, SCM_FOREIGN_LEN, SCM_FOREIGN_TYPED_P,
  SCM_FOREIGN_VALUE_P, SCM_VALIDATE_FOREIGN_VALUE, scm_foreign_set_x,
  scm_foreign_type): Remove.
  (scm_foreign_ref): Rename to...
  (scm_foreign_address): ... this.
  (scm_take_foreign_pointer): Update.
  (SCM_FOREIGN_POINTER): Remove CTYPE argument.  Update callers.
  (scm_make_pointer): New declaration.

* libguile/foreign.c (scm_to_uintptr, scm_from_uintptr): New macros.
  (scm_make_pointer): New function.
  (scm_take_foreign_pointer): Remove TYPE and LEN arguments.  Update
  callers.
  (scm_foreign_ref): Remove to...
  (scm_foreign_address): ... this.  Remove type-related code.
  (scm_foreign_set_x): Remove.
  (scm_foreign_to_bytevector): Change argument order; make LEN argument
  compulsory.
  (scm_i_foreign_print): Remove type printing.
  (unpack): Remove foreign-type checking.

* libguile/deprecated.c (scm_dynamic_args_call): Update accordingly.

* libguile/dynl.c (scm_dynamic_pointer): Remove the TYPE and LEN
  arguments; update callers.  Update to the new foreign API.

* libguile/dynl.h (scm_dynamic_pointer): Update.

* libguile/gsubr.c (create_gsubr): Update to the new foreign API.

* libguile/gsubr.h (SCM_SUBRF, SCM_SUBR_GENERIC): Ditto.

* libguile/snarf.h (SCM_IMMUTABLE_FOREIGN): Ditto.

* libguile/vm-i-system.c (subr_call): Ditto.

* module/system/foreign.scm (null-pointer?): New procedure.

* test-suite/standalone/test-ffi: Update to the new
  `bytevector->foreign' signature.

* test-suite/tests/foreign.test ("null pointer")["null pointer
  identity", "null-pointer? %null-pointer"]: New tests.
  ["foreign-set! other-null-pointer", "foreign->bytevector
  other-null-pointer"]: Remove.
  ("make-pointer", "foreign<->bytevector"): New test prefixes.