What version of Rust does Tor require for 0.3.2?

On #24761 (moved), Sebastian says that we require Rust 1.14, but I can't see that documented anywhere.

We should document it:

• in the release notes
• in the wiki: https://trac.torproject.org/projects/tor/wiki/RustInTor

And point to it from:

• https://gitweb.torproject.org/tor.git/tree/doc/HACKING/GettingStartedRust.md

And document the nightly/feature thing in our coding standards:

• https://gitweb.torproject.org/tor.git/tree/doc/HACKING/CodingStandardsRust.md