slingshot: sync with upstream.

* slingshot: Upgrade again now the we are maintaining API
compatibility in the next release of lyaml.
* bootstrap: Pick up fixes from latest slingshot.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
configury: add sanity-cfg.mk.

* build-aux/sanity-cfg.mk: New file. Add exception to casting
argument to free check.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
specs: remove trailing blanks.

* specs/ext_yaml_emitter_spec.yaml: Remove trailing blanks.

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