WIP: an XS interface to Lua tables

Bizarrely, this relies on:

- On creation, put the table in the registry (luaL_ref).
- Stash the registry index in the C-level struct with the lua_State
- DESTROY calls luaL_unref to let go.
- Actual accesses need to recover the table from the registry and then
  push the key on top, etc. Kind of worried about the performance of
  that, but a test implementation (which assumes string keys and
  integer values...) manages to do 0.65 as many accesses as a regular
  Perl lexical hash $h{foo}. Seems reasonable considering the XSUB
  overhead?
- To be useful, requires a generic "lua_value_to_sv" function that can
  convert values. Alas, Lua can handle anything as keys, so there's more
  interesting bits to come.