Restructure edgedb-protocol

I'd expose 4 public modules from edgedb-protocol:

1. model - Types an application will need in its model
   • BigInt, Decimal
   • Duration, LocalDatetime, LocalDate, LocalTime
   • Re-export of uuid::Uuid
   • TBD: what to do about Datetime vs SystemTime and json
2. dynamic/value - Value, NamedTuple and related types which are needed to represent unknown edgedb data at runtime
3. serialization - traits for serialization, de-serialization (currently Queryable) and related types
4. messages - probably should split this one into a separate crate (or rather move the rest to a separate crate like edgedb-data)

The issue names the edgedb-protocol crate and proposes public model, dynamic/value, serialization, and messages modules. Start by inspecting the crate's current module and export layout, then review how Value, NamedTuple, Queryable, and the listed model types are organized. Done requires an agreed module structure and a decision on the Datetime/SystemTime, json, and messages-crate questions.