RFC: Remove FormattedString field

[sloria]

fields.FormattedString only exists because it was ported from Flask-RESTful. It has a couple peculiarities:

1. It converts the object to a dict in a way that is error-prone and inefficient.
2. Its deserialization behavior is no-op, making it asymettrical with its serialization behavior.

fields.Function and fields.Method achieve the same use case in a more performant way.

from dataclasses import dataclass
from marshmallow import Schema, fields


class MySchema(Schema):
    formatted = fields.FormattedString("{first_name} {last_name}")
    function = fields.Function(lambda u: f"{u.first_name} {u.last_name}")


@dataclass
class User:
    first_name: str
    last_name: str


u = User(first_name="Steve", last_name="Loria")
data = MySchema().dump(u)
assert data["formatted"] == data["function"]

[lafrech]

#1046

This field achieves a function that could be achieved otherwise (property, pre_dump,...). I think it could be dropped without too much collateral damage.

[sloria]

If there are no objections to this proposal, I'll plan on merging #1142 at the end of next week.