🚀 Calculate code coverage

[marcoieni]

We could calculate code coverage percentage on every push and display it with a badge in the README.
For code coverage calculation I use https://github.com/actions-rs/tarpaulin
This creates an lcov.info file that I upload to https://coveralls.io/ (free for open source).

Setup example: https://github.com/rust-github/rust-gh-example

https://github.com/rust-github/rust-gh-example/blob/74be2688c366c78791fa52e5674bf317cdf45cbb/.github/workflows/ci.yml#L60

I solve this issue if you want.
Of course the maintainer has to register to the coveralls website.

[dandavison]

Sure! I've heard of coveralls but not used it. I've signed up and registered delta:

[marcoieni]

Closed by #409