Add lua_tolightuserdata, optimized lua_topointer

[petrihakkinen]

This change adds lua_tolightuserdata to the public API. Rationale:

First, it's faster than lua_touserdata when working purely with lightuserdata values. In our codebase we use lightuserdata for opaque 64-bit object handles (handles to objects managed by C++ code). With lua_touserlightdata we are seeing up to 8% performance improvement in Lua to C++ calls that use these handles. I believe this is a common use-case for lightuserdata.

Secondly, since the API already has lua_pushlightuserdata and lua_islightuserdata, it would make sense to have lua_tolightuserdata for completeness.

I also optimized lua_topointer a bit by reducing branching in userdata case.

[zeux]

This looks good! Do you think it's worth it to convert lua_touserdata to use two ifs so that for that function we prioritize regular userdata as a fast path?

[petrihakkinen]

Makes sense. Made the changes and a benchmark calling lua_touserdata 10 million times is now about 8% faster.