specl: support continued (undocumented!) specl-1 _spec.lua specs.

* src/specl.in (slurp): First try to load as pure Lua, and if
that fails, fall back to the YAML loader before erroring.
* src/specl.lua (copile_specs, compile_examples, compile_example):
Factor out compilation phase for YAML specs.
* build-aux/speclc.in: New file. %_spec.yaml to %_spec.lua
compiler (requires lyaml!).
* specs/specs.mk (build-aux/speclc): Build it from speclc.in.
(.yaml.lua): New rule to compile all yaml specs to lua to make
sure Lua specs continue to be fully functional.
(specl_SPECS): Add lua specs.
* build-aux/.gitignore: Add speclc.
* specs/environment_spec.yaml: Comment out a spec that fails
when loaded as a lua spec, but passes as a yaml spec :-?
* TODO.yaml: Remove done items.