Add extra validation when generating GraphQL fields

[ianhoffman]

https://github.com/mwildehahn/hack-graphql/pull/52/files#r632070440

A simpler option could be to just enforce that <<GraphQL\Field(...)>> can only be used once for each method name across the hierarchy -- e.g. if there's <<GraphQL\Field('foo')>> on an interface, adding <<GraphQL\Field>> on the overridden method in the subclass would be illegal.