specs: specify exit statuses

* specs/specl_spec.yaml (describe specl): New examples to specify
exit status of specl.
(describe matchers): Move everything else from here...
* specs/matchers_spec.yaml: ...to here.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
yaml: compile lua fragments on the fly instead of requiring `!lua`.

* yaml/lyaml.c (load_scalar): Remove `!lua` precompiling support.
* src/specl.lua (compile_example): New function to compile an
example.
(run_examples): Use it when encountering an uncompiled example.
* specs/environment_spec.yaml, specs/matchers_spec.yaml,
specs/specl_spec.yaml: Remove all `!lua` tags.
* specs/specl_spec.yaml: New expectations for yaml specs on CLI
standard input.
* README.md: Update documentation. Particularly removing the
notes about dealing with quirks of `!lua`.

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