Update how cargo talks to the registry

Update how cargo talks to the registry

This commit includes a laundry list of updates and tweaks to reflect the current
API of the registry:

* `registry.host` has been renamed to `registry.index`
* New top-level manifest keys are now accepted:
  * `homepage` - url
  * `documentation` - url
  * `repository` - url
  * `description` - a markdown-less blurb
  * `license` - string (verified by the registry on upload)
  * `keywords` - string array
  * `readme` - string pointing at a file
* Authors are now uploaded to the registry
* The upload format to the registry has changed to a body json payload
* Unpacking tarballs respects the executable bit for scripts and such.
* Downloading now follows redirects to go to S3.
* The download URL for a package has changed slightly.
* Verify path dependencies have a version listed when being uploaded
* Rename `upload` to `publish`
* Rename `ops::cargo_upload` to `ops::registry`
* Add a new `registry` package for interoperating with the registry
* Add the ability to modify owners via `cargo owner`
* Add a `readme` key to the manifest, and upload its contents to the registry.
* Add the ability to yank crates and their versions
* When packaging a library, verify that it builds from the packaged source by
  unpacking the tarball and simulate running `cargo build` inside of it.