Design and implement compiler opt pass into `add_n`

[mortendahl]

This issue is about writing a compiler pass that rewrites consecutive add (binary) ops into a single add_n (variadic) op.

Note that we don't currently have an established design for such passes, so part of this issue is also figuring out what we want there.

[kkloberdanz]

I'd be interested in working on this one when we think the time is right!