slingshot: let slingshot manage its own files.

* GNUmakefile, Makefile.am, build-aux/do-release-commit-and-tag,
build-aux/gitlog-to-changelog, build-aux/mail, build-aux/mkrockspecs,
build-aux/release.mk, build-aux/rockspecs.mk, build-aux/sanity.mk,
build-aux/specl.mk, m4/ax_compare_version.m4, m4/ax_lua.m4,
travis.yml.in: Remove.  Slingshot will pull back the right versions
during bootstrap.
* slingshot: New git submodule.
* bootstrap, bootstrap.slingshot: Manually copy in from Slingshot.
* .gitignore, bootstrap.conf, configure.ac: Adjust.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
travis: regenerate .travis.yml.

* .travis.yml: Regenerate from Slingshot.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
stdlib: update object implementation from upstream.

* lib/specl/std.lua (Object, new, object): Update to typed object
from upstream.
* lib/specl/util.lua (Object, typeof): Remove custom type layer.
(object, object.type): Re-export apis from "specl.std".
Adjust all callers.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>