Fix individual library open functions

[giodamelio]

See #146.

I think I got this mostly working, but since I have never programmed in C or used the Rust FFI before things could be totally broken.

The Lua 5.2 Docs say that you should use requiref to require individual libraries. So I basically implement this. My understanding of Lua internals is practically non-existent, so I hope this is right.

Just one part I'm not sure how to do cleanly. See the line comment below.