w05: Adding Parser and Ring
w05: Tweaks to Parser and Ring

- Restricted imports to suppress warnings
- Added a type signature for buildOp
w05e01: intParsingWorks
w05e02: Defining Mod5 and Ring/Parsable instances
w05e03: Defining Mat2x2 and Ring/Parsable instances
w05e03: Improving Parsable Mat2x2