refactor: use installed stdlib.

* lib/specl/std.lua: No need to carry around local copies of
stdlib pieces we use; load installed stdlib modules into the specl
namespace, and clear the package.loaded cache so that examples
can load and check behaviours of another version of stdlib than
the one specl loaded for itself.
Adjust all callers.
(std.io.dirname): Set a fallback for this function, not exported
by stdlib v40.
* rockspec.conf (dependencies): Add stdlib v40.
* lib/specl/optparse.lua: Remove.  Use the installed std.optparse.
Adjust all callers.
* local.mk (bin_specl_SOURCES): Remove lib/specl/optparse.lua.

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