Beginning of a custom error-resistant parser to rowan's CST

[kpagacz]

• Implemented beginnings of a hand-rolled left recursive descent parser with possible error recovery
• Parsing to rowan's CST
• Then lowering to AST
• Also implemented a view that maps to the results from the old parser
• Added a testing framework for comparing the results of the new and the legacy parser to verify the parity between the two

Implemented literal expression and binary expression parsing for now only.