Make the Lua struct an opaque

[natecraddock]

Follow up from #40

This will remove the state field from the Lua struct and make the type an opaque. This makes the code simpler and shows the intent of the Lua struct more clearly.

[natecraddock]

Just tried this. In some areas the code is more readable. In others, less so.

No impact on the output binary, seems like llvm can optimize the code the same regardless.

I will keep the changes in a branch and reevaluate at a later time