Nicer DSL syntax for Maybe/Set types (Maybe (Set UTCTime), Set Int)

[adinapoli]

Follow-up from PR #79 review (@adamgundry):

As we discussed, it would be nice to be able to write Maybe (Set UTCTime) for ? Set utc and similarly Set Int instead of Set integer (while keeping backwards compatibility for the old syntax). I suggest we save that for a follow-up PR though.

Goal

Extend the schema DSL parser to accept Haskell-style type syntax as an alternative to the current one:

Current syntax	Desired alternative
? Set utc	Maybe (Set UTCTime)
Set integer	Set Int
? T	Maybe T

Requirements

• Backwards compatible: existing schemas using ? T, Set integer, etc. must keep working unchanged.
• This likely means supporting the Haskell type constructor names (Int, UTCTime, ...) alongside the current lowercase DSL keywords (integer, utc, ...), mapping to the same APIType.