Summary:
Rust dependencies are vendored in by downloading a package containing all Rust
dependencies.
The package can be updated using the new `vendorcrates.py` script.
Test Plan: Build using the vendored packages.
Reviewers: quark, #mercurial
Reviewed By: quark
Differential Revision: https://phabricator.intern.facebook.com/D6689642
Tasks: T24908724
Signature: 6689642:1515548647:8051ec3dadd98873f0312bb67978846ba029b558