Squashed 'packages/mr/' changes from fd29e2b..dd39a7e


git-subtree-dir: packages/mr
git-subtree-split: dd39a7e4b175cec90c254e0d001e058c267c6d6c
Update Mr to 0.12.12
Promise disinherits Pledge

The promise library is a function now, so not a suitable base-class.  It
will need a more nuanced relationship to Pledge.