New high-level features for arti and arti-client

As we grow in complexity, running tests and whatnot under --all-features is increasingly ill-advised. (See for example discussion at !550 (merged).) We should introduce other high-level features that give you "all the good stuff" or "all the feature-like features" or something.

Possible candidates are:

• full: This would imply every non-experimental feature, but no non-default backends, and no acceleration.
• all-backends: This would add all supported TLS and Async backends.
• crypto-accel: This would enable all supported crypto-acceleration options.
• full-experimental: This would turn on all experimental APIs of any kind.

Do we want more? Are these about right?

I'm blocking on this for #441 (closed), since it has design implications there.