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/Postgres/README.md

@@ -66,7 +66,7 @@ var customer = await Find.ById<string, Customer>("customer", "123");
 // Find.byId type signature is string -> 'TKey -> Task<'TDoc option>
 let! customer = Find.byId<string, Customer> "customer" "123"
 ```
-_(keys are treated as strings in the database)_
+_(keys are treated as strings or numbers depending on their defintion; however, they are indexed as strings)_
 
 Count customers in Atlanta (using JSON containment):