feat: support enum types in fields

[carllerche]

Toasty does not currently support enum fields as reported in #280.

Initial PR: #299

TODO

• How to update only part of a nested structure/enum.
• extend update assignments to be able to do partial updates.
• How to use embedded fields in queries
• Enums
• Nested embedded types.
• Universal model builder type (unify create and update for root and embedded models) where fields are expressions. Maybe store the internals as a set of assignments?