Refinement types

[brendanzab]

This will be needed to distinguish the prefixes of variants in union types (#118), and will allow us to move some more things from doc comments into types in the OpenType spec.

I'm proposing the following implementation process:

• Add the syntax and type checking rules for: { <ident> : <type> | <boolean-predicate> }
• Add check for the boolean predicate while parsing, failing the current parse if it evaluates to false
• (Mid term) Add syntactic sugar for record fields with operator expressions
• (Long term) Add constraint solving (Constraints #105)

[brendanzab]

Useful resources:

• Liquid Types
• LiquidHaskell: Experience withRefinement Types in the Real World
• CVC4 (seems like a nice solver that we could use)