RC4 changes (#7)

- Add `In` and `InArray` comparisons
- Replace `Op` with `Comparison` (internal API, but was public)
- Spell out comparisons in `Field` constructor functions

Reviewed-on: #7

src/Tests.CSharp/PostgresDb.cs

@@ -1,4 +1,3 @@
-using BitBadger.Documents.Postgres;
 using Npgsql;
 using Npgsql.FSharp;
 using ThrowawayDb.Postgres;