refactor: simplify deprecation management.

Closes #73.
* specs/base_spec.yaml (DEPRECATED): Specify behaviours of an
improved internal deprecation API.
* specs/list_spec.yaml (elems, index_key, index_value, relems)
(reverse): Specify default deprecation behaviours.
* specs/string_spec.yaml (assert, require_version, tostring):
Likewise.
* specs/table_spec.yaml (clone_rename, metamethod, ripairs):
Likewise.
* lib/std/debug.lua (_DEBUG): Document new compat field.
* lib/std/base.lua (_DEBUG): Set from debug_init.lua.
(deprecate): Rename from this...
(DEPRECATED): ...to this, and improve API.
* lib/std/functional.lua (bind): Use it to mark the old bind
API as deprecated since release 39.
* lib/std/list.lua (elems, index_key, index_value, relems)
(reverse): Collect in a new section and deprecate with the
improved API.
* lib/std/string.lua (assert, require_version, tostring):
Likewise.
* lib/std/table.lua (clone_rename, metamethod, ripairs):
Likewise.
* build-aux/sanity-cfg.mk (exclude_file_name_regexp): Don't choke
on error specs in specs/list_spec.yaml
* NEWS (Deprecations): Collect deprecated API NEWS for this
release.

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