[rust-dev] tree maintenance

Hi, As part of preparation for 0.7 (due around the end of june / early july) we're going to be closing the tree for a while, fixing the lingering breakage that's snuck through 'auto' and accumulated on 'master', and greatly widening the 'auto' coverage such that this stuff doesn't build up anymore. For now, bors is turned off and the tree is closed. Fear not though: this should ultimately improve life for everyone. More coverage, better differential testing, less chasing ghosts. Thanks for your patience. It'll be back asap (along with a snapshot!) -Graydon