Add Cargo.lock

[dotlambda]

I want to package fastuuid for NixOS in https://github.com/NixOS/nixpkgs/pull/465375. This would be a lot easier if you added a lock file to the repo because that would allow building fastuuid reproducibly. Thus it would be nice if you removed https://github.com/fastuuid/fastuuid/blob/d33252258c7e9d2ece27fa18f5beca5ad71d9d55/.gitignore#L4 and committed Cargo.lock.