Adding cargo lock

[spacekookie]

This PR adds a Cargo.lock file to the repository.

This is recommended behaviour according to the official
Cargo documentation:

If you’re building a library that other packages will
depend on, put Cargo.lock in your .gitignore.
If you’re building an executable like a command-line tool
or an application, check Cargo.lock into git

This change also means the easy building of a NixOS package

[spacekookie]

Any news?