improve unity<->lua (callmethod) interface

[iatenothingbutriceforthreedays]

• ideally support passing arbitrary structs (or at least primitive types other than just strings, multiple args would be nice)
• the call to unity from lua is synchronous (blocking) right now, that should probably be an option you can choose (e.g. if you don't need the return value and just want to fire off a message to unity)

[iatenothingbutriceforthreedays]

Also can we implement rpc calls in the other direction (unity->lua)? Unsure if it's possible to call a bizhawk lua function from a separate thread