Correct FFI callback types and add constants

Correct extern fn pointers, and add Lua's pseudo-index constants
Modify spacing style

Replace tabs with space indentation
Add indexing for registry/upvalues
Add FFI conversion for LuaState
Add flag for non-managed LuaState objects

Non-managed LuaState objects will not be closed on the C-side when drop is called
Add wrapper for Lua -> Rust function calls