Compiler Milestone 2: More realistic FT subset

[SupunS]

The goal of this milestone is to support most of the language features needed for the FT transfer transaction and contracts.

FT (everything expect closures)

Preview Give feedback

1. Default functions - @SupunS
2. Pre-/post-conditions (difficult, complex, e.g. linking/chaining, default functions, linearization, etc.) - @SupunS
3. Casting (force-casting as!; Optional casting as? doesn’t seem necessary, but maybe also implement while at it)
4. Fixed-point literal (compile to constant pool + load)
5. Event emission (simple, try to reuse infrastructure (e.g. export) of runtime)
6. If-let (for FlowToken) @turbolent
7. Switch statement (for FlowToken metadata standard impl.)
8. Dictionary literal (compile + add VM instruction; replicate what we have for arrays) @turbolent
9. Some of the comparison operators (<, ≤, etc.)
10. Fixed-point numbers (UFix64 for balance of Vault; wait until interpreter refactor)

• Not needed:
  - Entitlements: purely static
  - Swap
  - Assignment with second value

While at it, also add some of the fundamental language features:

• Control flow:
  • if statement with optional (if-let)
  • switch statement (just compiler)
  • for-loop statement (more effort, given iteration over different iterators, iteration over references)
• Double check we are erroring for unimplemented functionality in compiler