Stdx - The missing batteries of Rust

I've filled out descriptions of the existing crates. Things I'd like to do next:

  • Add more crates based on the suggestions here
  • Add a usage faq. A guide with simple recipes for common tasks, hopefully that combine the libraries in interesting ways. Doubles as a test suite (*).
  • Get reexported macros working
  • Publish 0.102.0-beta.1

(*) Part of my ambition for stdx is to help with Rust regression testing. stdx provides a set of specific revisions of popular libraries that can be exercised by tools like crater and ctrs, and if we can also provide a test suite that exercises them all together, that is really useful for future compiler writers.