Research: Code coverage solutions

[mtlynch]

We've had a few bugs in the past several months due to accidentally disabling tests. These would be harder to miss if we integrated with a code coverage solution. I've also seen discussions in code reviews about code coverage, but we don't have good tooling to show granular details about code coverage.

We should research options for code coverage services.

Here are the ones I'm aware of:

• Coveralls: I've used it, and I found it fine but not great. It felt like they stopped developing it 6 years ago.
• Codecov: Looks promising, haven't tried it