Decide whether values and selection sets can be reused past end of field resolution

[zombiezen]

One thing I've kept unspecified with field resolver methods so far is the lifetime and mutability of the field arguments and selection set passed into the method. (I also haven't specified such bounds on Context, but there's more of a convention around that.) Value and *SelectionSet are intended to be immutable values, so at the moment, there's no harm in keeping a reference to one, but a future version of the library may want to reuse the objects for the purpose of alleviating GC load. If this is something that might be needed, documentation should note it before hitting 1.0.