rust private

Auto-toolstate management follow-up. Tracking comment: https://github.com/rust-lang/rust/issues/45861#issuecomment-367302777 * Fixed rust-lang-nursery/rust-toolstate#1, a proper link to the PR will be included. * Fixed rust-lang-nursery/rust-toolstate#2, a comment will be posted to the PR if the toolstate changed * Toolstate regression will be rejected at the last week of the 6-week cycle (currently entirely date-based). * Implemented https://internals.rust-lang.org/t/the-current-submodule-setup-is-not-tenable/6593, moved doc tests of Nomicon, Reference, Rust-by-Example and The Book to the "tools" job and thus allowed to fail like other external tools.