Write smarter changelog tool in Rust

[Manishearth]

Our changelog process still calls for manual collation, but there's the boring part (collating by crate and reformatting) and the interesting part (actually prioritizing items in the changelog).

I made the script do most of the boring work.

It produces a report that looks like this: https://hackmd.io/XKCDISO_RhmAH7Ib1zqhjw. The first section of the report contains changelog entries organized by crate. After that the report includes information on additional "notes" found in some PRs that were not a part of the changelog entries but should be considered in the final collation, as well as a list of PRs without changelog information and PRs marked N/A for spot-checking.

A neat thing is that this separates the github fetching from the changelog generation: so you can fetch github state once to JSON (you can even update state for individual PRs in the json), and then run the tool repeatedly.

It currently uses the gh tool because I didn't want to set up a proper login flow.

This is a run-every-few-months tool, so I would prefer to land this as is, with improvements happening over time, without extensive code style/structure review. The maintenance burden of such a tool is low, and if it stops working, it stops working; it is not critical to anything.

Changelog: N/A

Internal

[sffc]

Looks reasonable, without having reviewed with a great level of detail.

[Manishearth]

Oops, pushed before noticing the review.

[sffc]

Things in tools/ don't need a super detailed review. It seems fine. I agree with using gh to reduce Rust dependencies.