Code coverage

[notgull]

In my most recent PRs, I've added a significant amount of code that doesn't end up being tested on the common case. In order to make sure our tests function properly, it would be nice to have an automatic code coverage utility in order to make sure every branch of our code is tested properly.

It may also be a good idea to implement this for other crates in this organization.

[mamaicode]

It's possible to add codecov ( free for open source projects)
https://about.codecov.io/

Unfortunately from my research, they only support one branch nevermind