Add cargo-edit utility

[spacekookie]

Right now the cargo_utils.rs file only contains utilities for reading from the appropriate Cargo.toml. But we need to be able to edit dependency versions as well.

Adding a function that can take a list of changes (Dep with changed version maybe?) and apply them to a Cargo.toml for a sub-crate would be a big step in the right direction

[afaq404alam]

Hi @spacekookie :) I was thinking to start working on this. Can you please give me a use-case based on current structure of Dep and DepGraph, which will help me move forward with this. I would also like to know how this will be used in a workspace project.