Move compare into event-model.

[danielballan]

These functions introduced in bluesky/databroker#392 should be moved into event-model, and #97 should be incorporated.

The useful bit is compare(a, b) which takes in two document streams and compares the documents with gradually increasing specificity. Rather than getting a dump like "These two lists of many documents aren't the same---good luck spotting the difference!" you get much more actionable advice.

I suspect we can update 100+ tests through the projects, which individually implement some fraction of this functionality, to make them more robust, succinct, and useful.