Support arbitrary 🥁 collections

I would like to be able to derive(Arbitrary) on a custom struct that happens to contain various collections that aren't in std (e.g. DashMap, SmallVec). I could add optional dependencies on dashmap and smallvec directly to the arbitrary crate, but I'd prefer to do this more generically so that it works for any collection.

I tried to do that as follows:

impl<'a, T: Arbitrary<'a>, C: FromIterator<T>> Arbitrary<'a> for C {
    fn arbitrary(u: &mut Unstructured<'a>) -> Result<Self> {
        u.arbitrary_iter().collect()
    }
}

which didn't work:

 1  error[E0207]: the type parameter `T` is not constrained by the impl trait, self type, or predicates
     --> /Users/jyn/src/arbitrary/src/lib.rs:1158:10
      |
 1158 | impl<'a, T: Arbitrary<'a>, C: FromIterator<T>> Arbitrary<'a> for C {
      |          ^ unconstrained type parameter

I have two questions:

1. Before I spent more time on it, is this a feature you're interested in?
2. Do you have suggestions for how to get it working?

Start in lib.rs around the attempted Arbitrary implementation at line 1158 and review how arbitrary_iter is intended to be used. Investigate the unconstrained type-parameter error and the project's preferred API for supporting collections outside the standard library. Done should be defined as a feasible generic approach for custom collections such as DashMap and SmallVec.