add support for inheritance, deserializing data into its class without knowing which class it is beforehand

[cmcarthur]

this doesn't fix the kwargs before positional args problem, but it makes inheritance a little easier. namely:

• fixes caching so that you can properly generate schemas for inherited dataclasses
• adds support for a JsonSchemaMixin.from_dict({...}) that searches the available subclasses for a match

note that the second breaks everything if you have a JSON schema with additionalProperties: true and all defaults. if you have that registered, every call to JsonSchemaMixin.from_dict will return you that thing

we should probably set additionalProperties: false by default to address this

[beckjake]

we should probably set additionalProperties: false by default to address this

+1 on this... +1000 really

[cmcarthur]

@beckjake I actually think I should add those back in, because I think I broke all_json_schemas. what do you think about adding additionalProperties: false to all of our JSON schemas by default?

[cmcarthur]

@beckjake ready for another look