Added SpannedAggregator to common crate

[Modularius]

Summary of changes

Introduced SpannedAggregator trait to encapsulate the span-aggregating behaviour of digitiser_aggregator::Frame and nexus_writer::Run.

Instruction for review/testing

Ensure changes are as described.

[DanNixon]

On it's own, a trait with no uses or implementations is not really an atomic PR. What will it be used for? Is there a reason there are no implementations?

[Modularius]

I wasn't sure how you wanted me to split the big PR up. This will be implemented in nexus writer and aggregator in a future PR. I thought it best to implement them separately to cut the number of "files changed" down. I had thought it might be difficult to piece apart the implementation from the other changes, but in retrospect, it makes more sense to put the basic implementations in here.

[Modularius]

Note some changes had to be made to the FindSpan and FindSpanMut traits to get the same functionality, but those traits will disappear in a future PR so the extra complexity will disappear.