Introduce com.apollographql.api.Fragment

[martinbonnin]

Fragment is like Operation but for fragments: It binds fragment definitions to their variables so we can read fragments from the store in a type-safe, variable-safe way

Since fragments do not have variable definitions, variables are inferred from field argument types and cannot have a default value.

With this PR, a fragment DroidFragment on Droid will generate:

• DroidFragment: the interface
• DroidFragmentImpl: Fragment.Data: the implementation
• DroidFragmentFragment: Fragment: the typesafe wrapper

The FragmentFragment thing doesn't feel too nice. We can be fine tuned in a follow up PR