Include additional hashes in src/stage0

This patch changes `bump-stage0` to include:

* The sha256 hash of the channel manifest used to create `src/stage0`.
* The rust and rustfmt git commit in `src/stage0`.
* Hashes of all the artifacts, like the source tarball, in `src/stage0`.

Combined this will allow for:

* Projects that bootstrap their own compiler, such as Fuchsia, or users
  of [bootstrap], to build their compilers offline without needing to
  communicate with static.rust-lang.org.

* Auditors to detect if the channel manifest, and all the artifacts
  inside the manifest, were modified after it was used to generate
  `src/stage0`. Furthermore, if they did find modified artifacts, they
  could determine if the Rust Signing Key was compromised by checking if
  any modified file was signed properly.

Finally, it allows regeneration of `src/stage0` when specifying both the
day of the build for rust, and the day of the build for rustfmt, which
can allow a maintainer to regenerate `src/stage0` to verify nothing
changed.

[bootstrap]: https://github.com/dtolnay/bootstrap
[mrustc]: https://github.com/thepowersgang/mrustc

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