rustfmt source code and GitHub action

[cpmachado]

Before anything, I'm sorry for #47 and #46 , they were mistakes.

On something useful, I'm suggesting, as referred on HowToLearnRust.md:66, to use rustfmt.

I like tooling and rules to follow, and the means to follow them.

As part of my apology, I wrote this github action largely adapted from : https://github.com/actions-rust-lang/setup-rust-toolchain

On another note, I'm loving your crash course and will recommend. I'm in the middle, but loved to discover clippy, which was referenced in one of the exercises.

Best regards, cpmachado