dev-rust/rayon: Add a proper ebuild for rayon.

I'm using it in the new deqp runner, and it's a very popular crate.

BUG=b:171728575
TEST="emerge-trogdor deqp-runner"

Change-Id: I1b0ba363751fabe48bc9442fc34b28fff9a039d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/2521900
Tested-by: Eric Anholt <anholt@google.com>
Reviewed-by: Eric Anholt <anholt@google.com>
Reviewed-by: Chirantan Ekbote <chirantan@chromium.org>
Commit-Queue: Eric Anholt <anholt@google.com>
4 files changed