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Covariant Wrapping Types (#1136)

**Semi-breaking: Those using flow should change from `new GraphQLList(t)` and `new GraphQLNonNull(t)` to `GraphQLList(t)` and `GraphQLNonNull(t)`**

Because class constructors are contravariant since flow/js lacks a "finally" keyword, we need to use functions to represent covariance in GraphQLList and GraphQLNonNull.

By having wrapping types be covariant, we can enable more checks and have more accurate predicates for refining types in GraphQL utilities.

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