Support for shrinking/growing Vec<T>

[landaire]

When calling Mutatable::mutate on an object that contains a Vec, the current impl Mutatable<T> for Vec<T> will treat itself as a slice and mutate all objects in-place. Shrinking/growing the Vec is desirable and should, for a baseline, require the following work:

1. Fix mutate methods not passing down Constraints.
2. Add two new impls:

impl<T> Mutatable for Vec<T>
where
    T: Mutatable

Where only shrinking a Vec would be performed

and:

impl<T> Mutatable for Vec<T>
where
    T: Mutatable + NewFuzzed + SerializedSize

Where shrinking or growing a Vec would be performed. The separation here allows us to call NewFuzzed::new_fuzzed() on the new elements in the vec, while also respecting the max size constraint if it's provided.

Another desired feature would be to randomly mutate fields or slice elements to be the length of any known vectors in the data structure (including encapsulating structures), but this is a pretty large undertaking.