Added support for enums in inputs

[liamcurry]

Enums are already supported in results, but not as input values. This PR adds support for enums as input values.

For example, with a schema like this:

enum Fruit {
  APPLE
  ORANGE
  PEAR
}

type Query {
   getFruit(fruit: Fruit): String
}

schema {
   query: Query
}

This query should now work:

query getFruit($fruit: Fruit) {
  getFruit(fruit: $fruit)
}

This is working for my small project but please make sure I didn't miss anything. I'm happy to make changes if necessary so please let me know! Thanks.