Allow passing identifiers to lua_string!

[JoshPiper]

As title, allows passing identifiers as well as string literals, such as is used in gm_sysinfo

Unfortunately, however, the macro has not been tested to ensure operability, however, similar code is already used in larger macros, so it shouldn't be an issue.

[WilliamVenner]

I think this will be confusing to use.

let a = 50;
lua_string!(a) // Does the user expect a Lua string of "50", or "a"?

[JoshPiper]

Hmm, hadn't considered that. Perhaps some sort of implementable type later down the line would be more appropriate then.