Implement the [patch] section of the manifest

Implement the [patch] section of the manifest

This is an implementation of [RFC 1969] which adds a new section to top-level
manifests: `[patch]`. This section allows you to patch existing sources with
new versions of crates, possibly replacing the versions that already exist in
the source. More details about this feature can be found in the RFC itself.

[RFC 1969]: https://github.com/rust-lang/rfcs/pull/1969