Improve documentation around Cargo features; make Runtime require Debug

• #445 (closed) 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 #445 (closed)

requested review from @nickm

approved this merge request

LGTM but pipeline failed for what looks like network failure reasons. Retrying pipeline.

enabled an automatic merge when the pipeline for 864fd039 succeeds

mentioned in issue #445 (closed)

merged

mentioned in commit cc3ec808