Stability for core::cmp, core::default, alloc::rc, std::task

Summary:

* alloc::rc module stable
* Rc type stable
* Functions relating to weak references experimental
* core::cmp module stable
* PartialEq/Eq/PartialOrd/Ord unstable because trait reform will make them change again
* Equiv experimental because there may be better sol'ns
* lexical_ordering deprecated because it can be done trivially with the Ord trait
* min/max stable
* std::task module stable
* TaskBuilder::stdout/stderr experimental because we aren't certain we want to configure the environment this way
* try_future experimental because Future is experimental
* try unstable because the error type might change
* deschedule/failing unstable

The major thing I did differently than previously-discussed is that I made `try` experimental: there's been discussion that the error type `Box<Any + Send>` is not sufficient.


Per https://github.com/rust-lang/meeting-minutes/blob/master/Meeting-API-review-2014-07-16.md.
Inspecting the name is often a debugging utility, not done in performance critical contexts, so I don't think there's really any benefit in avoiding an allocation.

The only part that *may* be performance sensitive (not measured) would be spawning a task with a specific name and whether to special-case the `&'static str` case, but the actual inspection of the name seems much rarer.