Support customizing deserialization, optionals, and dicts

This allows customizing deserialization with DeserializeCustom, and adds builtin support for a number of additional types.

Features

• Add DeserializeCustom abstract class for customizing deserialization.
• Support deserializing dict[K, V]
• Support deserializing Optional[T]
  • Other Union types are currently unsupported
• Support default/optional fields in dataclasses

Fixes

• Fix accidental reversal of location keys
• Improve DeserError tracking of locations by accepting an explicit ctx parameter
• Move deserialization code into seperate deser module
• Support deferred evaluation of annotations (#1)

Full Changelog: v0.1.0-alpha.1...v0.1.0-alpha.2