LUA_LOADED_TABLE and friends

[digitcrusher]

It would be nice if constants such as LUA_LOADED_TABLE were available, instead of having to rely on LUA_LOADED_TABLE always being equal to "_LOADED". I think the following excerpt from the Lua reference manual is a good argument for this change:

Lua versions can always change the C API in ways that do not imply source-code changes in a program, such as the numeric values for constants or the implementation of functions as macros.

And besides, I think everyone can agree that lua.loaded_table looks nicer than "_LOADED". :-)

[natecraddock]

Good point! This fits in well to #35 so I made note of it there

[natecraddock]

I added LUA_LOADED_TABLE and LUA_PRELOAD_TABLE. Other missing constants can be added as needed, and if any are missing zlua.c can be used to access them directly.