0.18.3

• Snapshots of maps keyed by a fixed set of values (MapSchema<V, "a" | "b">, IMap<SomeUnion, V>) now make each key optional, so fixtures and mocks no longer need every key. Reading one directly (snap.traits.damaged.x) now needs a check or ?., as map.get() already did; string-keyed maps are unchanged. (#14, thanks @FTWinston!)