Add fields.Iterable as a base class for collection field types

Nobody has claimed this yet.

I'd like to deserialize into some types that aren't json serializable, for example a frozenset of integers, `frozenset({1,2,3})`.

I can think of a few options in the current API:

- define a custom `FrozensetField` type and define its `_serialize` and `_deserialize` methods,
- use `fields.Function(deserialize=frozenset, serialize=list)`,
- define a custom replacement for the json module.

Is `fields.Function` the recommended strategy here? I'm unsure because

- it doesn't take a `cls_or_instance` parameter like `fields.List` does, so it's not obvious how to make a `set` *of* some other objects. For example, how do I define a frozenset of `fields.Nested(OtherSchema)`?
- it makes `Function` the main focus of attention, instead of the data.

Alternatively, an API that comes to mind is something like

```python
class SomeSchema(marshmallow.Schema):
x = fields.Integer()
y = fields.String()

class MoreSchema(marshmallow.Schema):
values = fields.List(fields.Nested(SomeSchema), deserialize=frozenset)
data = fields.Mapping(
keys=fields.String(),
values=fields.Integer(),
deserialize=collections.OrderedDict
)
```

Like `fields.Function`, these might also take a `serialize` argument, defaulting to `list` and `dict`, respectively.

This would generalize the existing idea of [`Mapping.mapping_type`](https://github.com/marshmallow-code/marshmallow/blob/dev/src/marshmallow/fields.py#L1272) , and would make it an instance attribute rather than a class attribute.

What is the recommended approach for deserializing to `Frozenset[int]` or `Frozenset[MyObject]`? Do you think an API like the idea above would be useful?

Start with src/marshmallow/fields.py, especially fields.List, fields.Mapping, and Mapping.mapping_type. Compare the proposed collection-type API with fields.Function and determine the expected behavior for frozenset values and nested objects. The issue does not define a settled API, implementation location, or tests, so the desired interface and acceptance criteria need clarification first.

Written by the indexing model from the issue text.