Import `cargo fix` directly in to Cargo

Import `cargo fix` directly in to Cargo

This commit imports the `cargo fix` subcommand in rust-lang-nursery/rustfix
directly into Cargo as a subcommand. This should allow us to ease our
distribution story of `cargo fix` as we prepare for the upcoming 2018 edition
release.

It's been attempted here to make the code as idiomatic as possible for Cargo's
own codebase. Additionally all tests from cargo-fix were imported into Cargo's
test suite as well. After this lands and is published in nightly the `cargo-fix`
command in rust-lang-nursery/rustfix will likely be removed.

cc rust-lang/rust#52272