specs: update to cleaner Specl-2 YAML spec format.

* specs/getopt_spec.lua, specs/package_ext_spec.lua,
specs/string_ext_spec.lua, specs/table_ext_spec.lua: Rename from
these...
* specs/getopt_spec.yaml, specs/package_ext_spec.yaml,
specs/string_ext_spec.yaml, specs/table_ext_spec.yaml: ...to
these.  Reformat as YAML.
* Makeflie.am (SPECS): Remove old _spec.lua filenames, and add
new _spec.yaml filenames.