- arti#445 highlighted the lack of good documentation around Arti's
multiple runtime support, as well as it being difficult to determine
what runtime was actually in use.
- Improve the documentation to solve the first problem.
- To solve the second problem, make Runtime require Debug (which is
arguably a good idea anyway, since it makes them easier to embed in
things), and print out the current runtime's Debug information when
arti is invoked with `--version`.
- (It also prints out other Cargo features, too!)
fixes arti#445
eta
Nick Mathewson