TODO:
- Get all run-pass and run-fail tests passing resolve
 - Lots of fixes to do here
- Audit TODOs in codebase
- MIR Optimisations
 - Dead code eliminiation
 - Remove variables that are just assigned from arguments
- Convert enum variants to point to structs for the data type
 - Four types of variants: Unit, Value, Tuple1, Composite
  - TODO: Have value enums be a different structure to data enums
  - Tuple1 is a special case to reduce the cost of Some/Ok/Err/...
 - This may have to wait for a rustc RFC? (or can hide the structs for now)
- Clean up Span code to correctly handle nested spans
- Clean up AST
 - Almost done, just a few little niggles left
- Split arg patterns and arg types up for cleaner serialisation
 - Could be a good idea to do as part of changing HIR::TraitImpl to only contain the impl data
 - May not be too useful due to argument monomorphisation.
- Optimise typecheck.


- MIR: Unify Variable/Argument/Temporary LValue types
 - This should reduce the amount of code needed for validation, but will be a
   BIG change.



## Optimisations
- Argument propagation: replace assignments from Argument(_) if target is only
  written once
- Dead assignment removal (Delete `<val> = Use(<val>)`
- Tuple destructure removal
 - Spot `#1 = (...,)`, `#2 = (#1).n` where #1 is Write-once, borrow-none