Implement std::ops::Add for vectors of field elements

[divergentdave]

There are many places in the codebase where we zip or index through two vectors, adding one to the other. We could write blanket implementations of Add and AddAssign on vectors of field elements to get rid of some boilerplate. These could also check that the lengths of the addend vectors are equal, and panic otherwise to be defensive.

See: #300 (comment)

[cjpatton]

What's y our idea here @divergentdave? You can't just implement Add on Vec<F> because Vec is a foreign type, i.e., this doesn't compile:

impl<F: FieldElement> Add for Vec<F> {
    type Output = Vec<F>;

    fn add(self, rhs: Vec<F>) -> Vec<F> {
        todo!()
    }
}

We could do this wherever we have a wrapper type, like OutputShare or AggregateShare. Would that suffice to close this?

[divergentdave]

Good point. Implementing it on OutputShare and AggregateShare sounds good. We could also go through and replace for loops adding field vectors with calls to appropriate private helper functions.

[cjpatton]

Alrighty, here's my proposal: #1205

A couple of caveats:

• I decided to not implement Add and AddAssign for OutputShare and AggregateShare, since the Aggregator API already facilitates arithmetic on these. I suppose we could simplify the implementations of our concrete VDAFs a little, but I didn't bother.
• vec_assign_vector(a, b) panics if a is longer than b, but not if b is longer than a. The reason is that I wanted to be able to let b be an iterator, and there are cases where the iterator has infinite about. For example, in Prio3, b is a Prng. As far as I can tell, enforcing the length of b would require an allocation here, but I could be wrong.