Possibly defer InputLiteral serialization.

[obmarg]

Currently it happens at query building time. For performance this may be sensible, but it means I have no way to propagate errors unless I change a lot of APIs a bit more. This has led to at least one unwrap/except which could come back as a problem, particularly because the serializer it's using is quite picky about what it serializes. Could defer this to serde::Serialize time and it could provide a point to error. Or possibly I could do it in the Operation functions? Needs some thought, either way.