Added implementation of #[field(flatten)].

[azewierzejew]

Limited to on flattened struct.

Implementation of #1916

[azewierzejew]

I'm proposing here an implementation (without error handling) of the flatten attribute in FromForm derive macro.
Before finishing it I would like to discuss the semantics of this trait.

With current FromForm trait interface, there is no way which flattened sub-struct uses given field.
One solution for that would be to pass the field to all flattened structs.
But that doesn't really work either because DataField can't be copied.
Also that would produce different results than serde for names duplicated by flattening, e.g:

#[derive(FromForm)]
struct X {
    name: String,
}
#[derive(FromForm)]
struct Y {
    #[field(flatten)]
    inner1: X,
    #[field(flatten)]
    inner2: X,
}

In above case serde initializes inner1.name with provided value and inner2.name with default value (or gives an Error).

Therefore I believe restricting flattening to only one field is the only way without serious API changes.