Add mutateIfFulfilled functions

[AnthonyMDev]

This addresses the limitations described in apollographql/apollo-ios#3443. This is solved by adding a new mutateIfFulfilled function that only allows for the properties of an inline fragment to be mutated if the fragment already is fulfilled, while still preventing a fragment from being added/removed from an existing model.

[svc-apollo-docs]

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[calvincestari]

Will these extensions cover named fragments too?

[calvincestari]

I don't think it matters to this particular test suite but it looks like we're missing the age selection.

[calvincestari]

Also, the test failures look like proper errors.

Error: cannot find type 'ApolloAPI' in scope
  mutating func mutateIfFulfilled<T: ApolloAPI.InlineFragment>(
                                     ^~~~~~~~~
Error: 'RootEntityType' is not a member type of type 'T'
  ) -> Bool where T.RootEntityType == Self.RootEntityType {
                  ~ ^
Error: cannot find type 'ApolloAPI' in scope
  mutating func mutateIfFulfilled<T: ApolloAPI.InlineFragment>(
                                     ^~~~~~~~~
Error: 'RootEntityType' is not a member type of type 'T'
  ) -> Bool where T.RootEntityType == Self {
                  ~ ^