Add Nix support for reproducible build and develop

[SnO2WMaN]

Add some files to provide a reproducible build and development environment (Rust tools, Python3, etc.) with Nix.

If Nix is installed, $ nix-build will generate binaries in result/bin/erg directly under the project, or in the same location as $ nix build if Nix Flakes is supported.

For this purpose, I've made Cargo.lock managed in git, according to https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html

If you’re building an end product, which are executable like command-line tool or an application, ... check Cargo.lock into git

[SnO2WMaN]

If necessary, rustfmt can be included in the development environment, although commented out the feature because currently about 5 files are not formatted.

#5

[mtshiba]

Looks good. Thank you so much!