Refactor ExprType

[Noah Charlton]

Currently ExprType is defined as https://github.com/DevOrc/aurora/blob/f0146452fb8ce9772162928e855c7b1841ff716a/src/lib.rs#L99-L102

It should probably refactored into the following:

#[derive(Debug, PartialEq, Clone)] 
 enum ExprType{ 
    Data(LuaData), SingleValue 
 }