Add make check-rustfmt to make check
In #26972 (moved), we discovered that stable and nightly rustfmt disagree about the formatting of some of Tor's Rust code. Beta currently follows nightly.
On 13 September, the current beta branch will become stable, and the formatting differences should go away. (Rust releases happen every 6 weeks, and the last one was on 2 August 2018: https://github.com/rust-lang/rust/blob/master/RELEASES.md .)
Here's what we need to do:
- cherry-pick the final commit on the branch 'rustfmt-travis' from https://gitgud.io/onionk/tor.git or https://github.com/teor2345/tor.git
- run rustfmt on any Rust code added since #26972 (moved), and commit the results
- make sure "make check" passes with Rust stable, beta, and nightly
- merge the changes
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