Field: respect single responsability principle, don't access parent object

Nobody has claimed this yet.

I have the feeling we could simplify `Field._serialize` signature from `_serialize(self, value, attr, obj, **kwargs)` to `_serialize(self, value)`.

From a quick search, `obj` is used in `Nested` to populate the `ValidationError`. I need to get my head around this. Shouldn't it be `nested_obj`?

It is also used in `FormattedString`. This field achieves a function that could be achieved otherwise (property, pre_dump,...). I think it could be dropped without too much collateral damage.

There may be an issue with `Function`, too.

I didn't see where `attr` and `kwargs` are used.

It boils down to whether a field should have access to the whole structure or only its own value.

This is more or less related to https://github.com/marshmallow-code/marshmallow/issues/799 and probably #900/#940.

Edit: Marking as 3.0 because it is a breaking change, but it could be postponed, just like #799. If my intuition is correct, it could make for a simpler and saner interface, forbidding hard-to-debug corner cases.

Start by tracing Field._serialize and its uses in Nested, FormattedString, and possibly Function. Review the related issues #799, #900, and #940 to determine whether the proposed breaking API change is agreed upon. Done means an explicit 3.0 decision about whether fields may access the parent object and which arguments remain.

Written by the indexing model from the issue text.