Lint: add rustfmt config and CI workflow

[zoedberg]

this PR adds a .rustfmt.toml file for rustfmt configuration, lints code, adds Lints github workflow and lint badge to README.

Is it ok to use nightly for the workflow? Should I switch to stable?

[dr-orlovsky]

Thank you! For linting we always use the nightly fmt, but I think you are right, since not each nightly releases fmt tool and it frequently breaks. So the best will be to stick to some specific rust version (and even not stable, since it also changes with time).

[dr-orlovsky]

Btw can we also add build CI, something like https://github.com/LNP-BP/rgb-node/blob/master/.github/workflows/build.yml?

I understand that it will cover FFI lib only, but it would be nice to have, especially with lipo and target-specific build matrix, if possible. Feel free to do it as a separate PR

[zoedberg]

So the best will be to stick to some specific rust version (and even not stable, since it also changes with time).

switched to rust 1.47.0

Btw can we also add build CI

yep, I'll do that in a separate PR