Add code coverage report/banner

[pool2win]

Seeing good code coverage will give confidence to other developers to use tower. It might also help find some bugs as we strive to improve coverage.

[seanmonstar]

Thanks for the suggestion. But I find code coverage tricky to count and report. In my experience, they tend to fail to notice various lines were covered. Many projects in Rust don't bother, and I don't think tower should either.

[GlenDC]

Code coverage as a metric of quality is pretty flaky. It can however be a nice tool in the back of a shed for maintainers to know if they might have some holes that they may wish to cover. But don't think it really is something an "outsider" should take as a mark of quality.

FWIW using something like tarpaulin it is fairly trivial to add a GH action for it :)