slingshot: add slingshot as a git submodule.

* .gitmodules: New file.
* slingshot: New submodule.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
configury: adopt slingshot release mechanism.

* GNUmakefile, Makefile.am, build-aux/mkrockspecs,
m4/ax_compare_version.m4, m4/ax_lua.m4, m4/ax_with_prog.m4:
Remove.  These files are handled by slingshot now.
* bootstrap: New file. Copied from slingshot subproject.
* bootstrap.conf (stdlib_copy_slingshot): Install missing slingshot
files.
* configure.ac: Adjust to drive slingshot processes.
* local.mk: New file.  Project local make rules.
* rockspec.conf: New file.  Slingshot rockspec template.
* specs/specs.mk: Simplified.
* .gitignore: Update.
* .travis.yml: Regenerate.
formatting: fix whitespace errors flagged by slingshot syntax-check.

* INSTALL, specs/debug_ext_spec.yaml: Remove trailing blank lines.
* specs/io_ext_spec.yaml, specs/match_ext_spec.yaml,
specs/table_ext_spec.yaml: Remove trailing spaces.
* std/std.mk: Remove SPACE-TAB sequence.
maint: collect previous release notes into NEWS.

* NEWS: new file, required for the Slingshot release process.
* local.mk (old_NEWS_hash): Update.