rust_tools: Add a tool to automatically generate a Rust uprev

This is the first pass to add a tool to convert rust upgrade
process into a program that can generate CLs autoamatically.
The tool is resumable, so when patches fail to apply, the user
can fix the patch, resume the tool until no problems exist.

The current tool only supports uprev from the earlier Rust ebuild
in chroot and will need future passes to add more features.

BUG=chromium:1112551
TEST=unittest;generated a CL locally

Change-Id: I77fccd14c69548824a8b235d756357aee0c42ef2
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2339593
Commit-Queue: Tiancong Wang <tcwang@google.com>
Tested-by: Tiancong Wang <tcwang@google.com>
Reviewed-by: Bob Haarman <inglorion@chromium.org>
Reviewed-by: George Burgess <gbiv@chromium.org>
2 files changed