lua_pushuserdata() polyfill

Sometimes, one needs to push existing userdata by pointer. This is not
safe (as the pointer might be stale), therefore it is not part of core
Lua api.

This header file should be included after Lua headers - mainly because
it overrides lua_newuserdata() with a macro, all hell would break loose
otherwise.

After that, you can push userdata by pointer. In case the userdata
object has been collected in the meantime, nil is pushed on stack.

This is handy for debugging, with native function you'd get dangling
pointer on Lua heap. It is also very slow, so it is not a good idea
to use this in release / performance bound code.

Patching Lua(JIT) with native function is left as an excercise for the reader.

-- k