Improve bug coordination & squash merges

[goomrw]

          If we do much more work in this repo it might be a good idea to adopt some practices like:

• having a way of tracking issues or easily correlating a Github PR to a b/ (ideally bidirectional)
• squashing PR commits down to a single one so the commit history is more readable/helpful.

These are some things Calcite does that I think work to its advantage, what do you think @goomrw @fzakaria ?

Originally posted by @tanclary in #185 (comment)

[goomrw]

I like both of these ideas! I'll do some investigation

[goomrw]

I've arranged to mirror issues & PRs into Google's internal tooling!
I'll talk to a few other contributors before changing the merge settings.