auto merge of #11263 : niftynif/rust/btree, r=catamorphism

Apologies for junking up the feed with all of these separate pull requests.  I'm still getting the hang of git and will hopefully be doing less of this nonsense soon.  I opened up another PR and closed the one from earlier today because the first PR was coming from the wrong branch of my repo.
Anyway, this contains a fleshed-out implementation of TotalEq/TotalOrd/Clone/ToStr for the whole B-tree structure and relevant tests, integrating suggestions and comments from several community members.
r? @catamorphism

Author: bors