Add combinator for optional properties

[edsko]

(Note that the implementation does not rely on defaultValue and hence does not rely on any Eq instances.)

[edsko]

Note that if we had the standard StackPrisms for Maybe, then we could just write

optProp :: Json a => Text -> Grammar Obj t (Maybe a :- t)
optProp propName = fromPrism just . prop propName <> fromPrism nothing