Update rave-level to be compatible with abstract-level 2.0 API

[Eranziel]

This PR updates rave-level to conform to the abstract-level 2.0 APIs. It requires Level/many-level#13. Currently it installs our fork of many-level, but of course that should be updated to use the official npm package once it is ready.

In general, this means everything has been converted from callbacks to promises. Unfortunately it does not encompass all of the work required to update rave-level; our use case does not use all of the features of rave-level, so due to limited time and competing priorities I had to forego solving several test failures which do not affect our use case.

A few notable changes:

• I updated the minimum node version to 16, matching abstract-level 2.0.
• This branch also includes a couple of bug fixes we implemented on our fork for rave-level 1.0.
• I have attempted to conform to the prevailing code style, but did add descriptive comments in several places. I hope these aren't an issue, because personally I would have found them quite helpful when gaining an initial understanding of the code base.
• There are many updates still needed to the test suite. Unfortunately I ran out of time to work on those.

While this isn't a complete update in and of itself, I hope this serves useful on the road to 2.0!

[Eranziel]

@vweevers hi there! Just wanted to touch base on this and see if you have any questions for me about this PR, or if there's anything I can do to help move it along? Not trying to be a bother, just checking in!