Make generating Cargo.nix in derivation easy

[kolloch]

Advantages:

• The user does not have to regenerate Cargo.nix when another dependency from crates.io is added (since we can use the hash from the lock file).
• The "opaque" package IDs will still contain local paths but they are the same for every user with a standard /nix/store location. So this solves Regeneration of Cargo.nix files for testing depends on local paths #47

This assumes that all hashes that need to be prefetched (git for now), have already been prefetched.

[kolloch]

The tools.nix provides an API for doing this. I am adding documentation.

It does not work yet with sub modules. Creating another issue for that.