Make FluentArgs a trait?

[dhardy]

FluentArgs's key API (beyond construction) would appear to be get. Maybe also iter; I don't know if that's actually needed.

Can we replace the type with a trait?

pub trait Arguments {
    fn get(&self, key: &str) -> Option<FluentValue<'_>>;

    // if necessary:
    // type Iter<'a>: Iterator<Item = (&'a str, FluentValue<'a>)> + 'a;
    // fn iter(&self) -> Self::Iter<'_>;
    // (Later this can just use impl Trait: see RFC#3425.)
}

(Technically, FluentArgs and fluent_args! don't need to change, but FluentBundle::write_pattern, format_pattern should use the trait.)

Motivation: this allows any type implementing fluent::Arguments to be used as arguments (often just via a derive macro). So, say, a Druid widget FluentText: druid::Widget<T> with data: &T could directly use that data type as arguments.

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Edit: return FluentValue by value not ref to allow construction in get and Iterator::next.

[dhardy]

Catch: in some cases the data might be a simple type, e.g. i32.
Solution: provide implementations of Arguments for such types using a fixed key ("value"?).