Deterministic builds

[RatCornu]

Closes #17.

• I added a flake.nix file (and a necessary associated flake.lock) for nix users with a devshell and a package
• I added rust-toolchain.toml and rustfmt.toml to set respectively the rust toolchain and options for cargo fmt
• I added a format check in the github workflow test
• I formatted the whole project using cargo fmt

I know that you mentioned developing on stable channel, but I encountered a little issue: it's not possible to use most features of rustfmt declared in the file rustfmt.toml on stable channel. That's why I chose to use nightly, but if you prefer to stick on stable I'll change it without problem!

[RatCornu]

Sorry I did something wrong in the commit tree ^^', I open a new PR