TODO:
- Get all run-pass and run-fail tests passing resolve
 - Requires getting libtest to compile
- Audit TODOs in codebase
- MIR Optimisations
- HIR Optimisation (remove dead code in `if true/if false` branches)
- Convert enum variants to point to structs for the data type
- Clean up Span code to correctly handle nested spans
- Clean up AST
- Explit arg patterns and arg types up for cleaner serialisation