[phobos] We desperately need to state the stability of each module in Phobos

[Don Clugston]

I suggest that in the ddoc at the top of each module, we add a section
named something like "Stability:" or "Development Status:"
This section can state anything from "Highly experimental, subject to
change without notice" to "stable, non-breaking changes only"
to "scheduled for deprecation, will be marked as deprecated in
DMD2.060 and removed in DMD 2.070. Use std.newstuff instead.".
If we do this, we can discourage users from relying too much on
unstable parts of Phobos.
(Ie, if somebody uses something marked as highly unstable, and their
code breaks, it's their problem. If it was marked as stable and it
breaks, it's our problem).

The current, rather ridiculous situation, is that we have users
assuming that everything is equally stable (stifling Phobos
development) or assuming that everything is equally unstable (making
serious use of D2 impossible).