add "deprecated" and "staging" packages and deprecate some modules

This MR adds two new packages to the Iris repository:

• The iris_staging package contains libraries that are not yet ready for inclusion in Iris proper. No stability guarantees whatsoever are made for this package. Each library comes with a comment indicating its status.
• The iris_deprecated package contains libraries that have been removed from Iris proper, but are kept around to give users some more time to switch to their intended replacements. No stability guarantees whatsoever are made for this package.

"deprecated" is meant to help with #378 (closed),though it doesn't really provide a permanent place for code that we want to keep but not see used by people.

"staging" is meant to help contributors: it provides a way for us to land a library and make sure it keeps building without doing all the work of making it "production-ready" all at once. This enables further incremental improvement of the library in question, either by the original author or by other people interested in it. This is inspired by the "staging" concept in the Linux kernel. The status comment at the top of the file should say what work needs to be done for this file to become ready for prime time.

I discussed this with @robbertkrebbers at our meeting yesterday. @jjourdan @tchajed we'd be interested in your thoughts on this.