refactor: lift lua compatibility functions into new module.

* lib/specl/runner.lua (loadstring, setfenv): Move from here...
* lib/specl/compat.lua: New file. ...to here.
Adjust all callers.
* local.mk (dist_noinst_DATA): Add lib/specl/compat.lua.

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