Implement installing Perl sub from lua_function

Missing bit is now only that I am currently installing the wrong Lua
function (I think) into Perl space. Cf. this TODO in the code in
plu_parse_kw.c:

/* TODO I think this needs executing once, then capturing a reference
 *      to the named function that has appeared in Lua global namespace,
 *      then using THAT to build the Perl coderef! */