First version of $foo.int to get Lua ints from Perl lexicals

This is some seriously nasty code with a number of shortcomings,
but there aren't "proper" ways of doing all of these. To support the
syntax, we do the following (beyond what's already there):

At Perl compile time:
- Install a "var_to_int" function into the Perl namespace in Lua. That
  function will take a PAD offset to find the lexical as argument. It
  will return that SV cast to a lua_Integer.

At Lua compile time:
- Pass Lua code to plu_munge_lua_code.
- From there, call perl function to extract all bits of the form
  $foo.int. Return hashref of the lexical names.
- Do all PAD lookups in C.
- Call perl function to replace the nasty syntax with a call to
  perl.var_to_int(X) where X is the PAD offset of the lexical.

Yeah, I know. Among other things, the actual scanning is done using
simplistic regular expressions. Calling into Perl is done only for
convenience to be able to use said regular expressions.