core: Stabliize core::cmp

Mark Eq, PartialEq, Ord, PartialOrd as unstable: they will change
slightly after trait reform. Equiv as experimental: better solutions
are desired. min/max stable.
std: Stabilize default

All stable.
alloc: Stabilize rc module.

Weak pointers experimental. Most everything else stable.
std: Stabilize task module.

Most stable. deschedule/failing experimental because of concerns about
naming and desirability.

Adds task::name() to replace deprecated task::with_name().
Fix typo.
auto merge of #15797 : brson/rust/taskstab, r=alexcrichton

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.
auto merge of #15803 : omp/rust/master, r=alexcrichton

Fix small typo in guide.
Moved `syntax::ext::base::SyntaxEnv` into `syntax::ext::base::ExtCtx`
Refactoring: Only use `MacroExpander` for expanding outside of
`syntax::ext::expand`