Generic enum

[teh]

Rough implementation, has some nice properties though not encoded as well as they could be:

• calling enumValues fails if there is an invalid name (right now through an incomplete pattern match). enumValues is mostly internal so we can decide how we want to fail here. Should tie in with general validation.
• Checks for unary constructors and doesn't compile if it's not. (Try instance GraphQLEnum A)

[teh]

Two examples:

instance GraphQLEnum B

data B = B1 | B2 | B3 deriving (Generic, Show)
data A = A1 | A2 Text | A3 deriving (Generic, Show)

[teh]

Thanks! I'll merge but will keep in mind that we need to fix enumValues.