slingshot: sync with upstream.

* slingshot: Pick up recent improvements, particularly compatibility
with upcoming lyaml 4 release.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
parser: set scalar style field.

* ext/lyaml/parser.c (parse_SCALAR): Set style field.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
load: replace C implementation with simpler Lua load.

* specs/lib_yaml_spec.yaml: Specify loader behaviour.
* ext/lyaml/lyaml.c (Load_Set_Metatables, Load_Numeric_Scalars)
(Load_Nulls_As_Nil, LUAYAML_TAG_PREFIX, RETURN_ERRMSG)
(lua_yaml_loader, generate_error_message, delete_event, do_parse)
(handle_anchor, load_map, load_sequence, load_sscalar, load_alias)
(load_node, load, Pload): Remove.
(handle_config_option, R): Adjust.
* lib/yaml.lua: New file. Reimplement in Lua.
* NEWS: Update.

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