sum-types: Support random testing for enums

[RyanGlScott]

On the sum-types branch (#1602), it is not currently possible to randomly test expressions involving enums, e.g.,

enum T = MkT

f : T -> Bool
f _ = True

Main> :check f
cryptol: src/Cryptol/Testing/Random.hs:(169,7)-(172,37): Non-exhaustive patterns in case

At a minimum, we should make Cryptol produce a proper error message here. We should also consider implementing random testing support for enums like we do for other types. We'll need to think carefully about what distribution of constructors is desirable. For instance, if we have:

enum Maybe a
  = Nothing
  | Just a

Then it is arguably more desirable to generate more Just values than Nothing values. Perhaps there is a simple way to encode this metric in the enum distribution that we use.

[RyanGlScott]

Support for random testing of enum values was added in 9438d83.