Allow unsafe direct access to Lua state

[jugglerchris]

We should:

• Add a way to get an ffi::lua_State pointer from an existing Lua.
• Add a way to construct a Lua from an externally created ffi::lua_State

Both should be unsafe in the Rust sense.

As part of this we also need to:

• Document what is safe or not
• Stop using the Lua "extra data" (from lua_getextraspace())

See #137 for some discussion.

[khvzak]

This feature is supported by mlua, see Lua::create_c_function to get access to raw lua_State. mlua-sys can be used to utilize raw C api in mlua.

Lua::init_from_ptr can be used to create mlua instance from a raw state.