Revert foreign objects.

For a long time the API failed to reach consensus among maintainers.
See <https://lists.gnu.org/archive/html/guile-devel/2015-11/msg00005.html>
and <https://lists.gnu.org/archive/html/guile-devel/2014-04/msg00069.html>.

This revert intends to break the deadlock and help further discussion to
take place with less pressure.

* libguile/foreign-object.c, libguile/foreign-object.h: Remove.
* libguile/Makefile.am (libguile_@GUILE_EFFECTIVE_VERSION@_la_SOURCES)
(modinclude_HEADERS): Adjust accordingly.
* libguile/init.c (scm_i_init_guile): Remove call to
'scm_register_foreign_object'.
* libguile.h: Remove inclusion of "libguile/foreign-object.h".
* module/system/foreign-object.scm: Remove.
* module/Makefile.am (SYSTEM_SOURCES): Adjust accordingly.
* test-suite/standalone/test-foreign-object-c.c,
test-suite/standalone/test-foreign-object-scm: Remove.
* test-suite/standalone/Makefile.am (check_SCRIPTS, check_PROGRAMS)
(TESTS): Adjust accordingly.
(test_foreign_object_c_SOURCES, test_foreign_object_c_CFLAGS)
(test_foreign_object_c_LDADD): Remove.
* doc/ref/libguile-foreign-objects.texi: Remove.
* doc/ref/api-foreign-objects.texi: Remove.
* doc/ref/libguile-smobs.texi: New file.
* doc/ref/Makefile.am (guile_TEXINFOS): Adjust accordingly.
* doc/ref/api-control.texi, doc/ref/api-smobs.texi,
doc/ref/api-utility.texi, doc/ref/guile.texi,
doc/ref/libguile-concepts.texi, doc/ref/libguile-program.texi:
Revert d9a4a1cd and 6e4630e0.

libguile/init.c

@@ -401,7 +401,6 @@ scm_i_init_guile (void *base)
   scm_bootstrap_vm ();
   scm_register_r6rs_ports ();
   scm_register_foreign ();
-  scm_register_foreign_object ();
   scm_register_srfi_1 ();
   scm_register_srfi_60 ();
   scm_register_poll ();