Consider moving `marshmallow-oneofschema` into the core library?

OneOfSchema is the go-to answer for questions about polymorphism.

I would like to see it on-course to have the following two things happen:

1. Avoid overriding load and dump, so that hooks work

As I have proposed elsewhere, I think it should hook in at _deserialize and _serialize. It's hard to make the case for extending marshmallow to support this use-case right now.

2. Built-in support in apispec

This is possible without bringing it into marshmallow, but harder to justify.

The current marshmallow-oneofschema code is quite small in size, only a little over 100 LLOC.
Plus, I think we can make it even smaller and simpler.

We could boil the whole thing down to something like this:

class OneOfSchema(Schema):
    oneof: Dict[str, Type[Schema]] = {}
    discriminator: Optional[str] = None
    def discriminate_on_load(self, data):
        return self.oneof[self.discriminator]
    def discriminate_on_dump(self, data):
        return self.oneof[data["discriminator"]]
    def _serialize(...): ...
    def _deserialize(...): ...

I also think moving it into marshmallow would result in more active maintenance.

@sloria, @lafrech, @deckar01, has this been discussed in the past? I wasn't able to find any useful clues other than some comments about not wanting to do this in the past. I understand not wanting to grow the public surface of a package and take on the related doc, bugfix, Q&A, etc workload, but in this case I think it's counterproductive.

I'd be happy to work on this if it stood a chance at being accepted.

Start by reviewing the existing marshmallow-oneofschema implementation and marshmallow's _serialize and _deserialize hooks, then assess the proposed apispec support. Done would require an accepted design and agreement on whether this functionality belongs in the core library; the issue names no files or tests.