Add return values to luacompiler.

[rbartlensky]

This PR implements the compiler side of multiple return values. The compiler emits two new instructions:

• ret - which returns to the parent function
• movr - which copies a return value from the stack into a specified register

The compiler also unpacks return values.

[rbartlensky]

I addressed all comments, ready for another review.

[ltratt]

Please squash.

[rbartlensky]

Done!