Struct as map representation and leftover field collection

[mitsuhiko]

This PR is similar to #1177 in spirit but very different in implementation. Because of current restrictions that serialize_field on the struct visitor requires static strings and because there is a hint about the incoming field names on the struct visitor I think this "collect other things" feature currently only really works when serializing as maps.

I tried a different approach now which also adds a neat additional feature. A (struct) container can now be marked with #[serde(repr="map")] which serializes the struct not as a struct but as a map. This then also unlocks #[serde(unknown_fields_into="...")]

So the example looks slightly modified like this:

#[derive(Deserialize, Serialize, Debug)]
#[serde(unknown_fields_into="other", repr="map")]
pub struct Breadcrumb {
    #[serde(rename="type")]
    ty: String,
    timestamp: f64,
    other: HashMap<String, String>,
}

Unlike the other one this however has a chance of being merged I think because it can be made sound and user friendly without requiring changes to the already existing traits. In a far serde 2.0 future the repr="map" requirement could then be removed.

Known issues:

• calls to_string which does not work in no_std. Generally this right now does not work in no_std mode. That might be okay if it's shimmed properly.
• I don't like the fact that I added a second bool to some methods.
• duplicated key checking is missing
• needs tests
• the branch that decides on __value decoding is complex. Could need some refactoring

[mitsuhiko]

Closing this. Working on a version of this PR with a coworker that actually implements flattening instead. This turns out to be what we actually need for most of our usecase anyways. Will push out a new PR soon.