refactor: compile lua examples during spec loading.

* lib/specl.lua (macro, compile_example): Move from here...
* lib/specl/loader.lua (macro, compile): ...to here.
* lib/specl.lua (reserved, compile_specs, compile_contexts)
(compile_examples): Remove, no longer required.
(run): Remove precompilation pass.
* lib/specl/loader.lua (parser_mt:load_map): Compile string type
map values upon loading.
(parser_my:load_sequence): Compile befores and afters during
hoisting.
* lib/specl.lua (run_examples): Only run befores and afters if
they are available.

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