scanner: bind YAML tokenizer.

* specs/ext_lyaml_scanner_spec.yaml: New file.  Specify behaviour
of tokenizing scanner.
* ext/lyaml/scanner.c: New file. Implement binding of scanner.
* ext/lyaml/lyaml.c: Declare scanner entrypoints.
(R): Register new scanner method.
(luaopen_lyaml): Call setup for __gc scanner cleanup.
* local.mk (ext_lyaml_lyaml_la_SOURCES): Add ext/lyaml/scanner.c.
* README.md: Add instructions for using lyaml.scanner().
* NEWS: Update.

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