API change: rust_kernel::create_scheduler() or
rust_scheduler::rust_scheduler() respecitevly now take ownership of the
launch factory argument, it is needed to create new threads on demand.

Also renames rustrt::sched_threads() to rustrt::rust_sched_threads() for
consistency. Added rustrt::rust_max_sched_threads() to return the
maximal number of scheduled threads of the current scheduler.

Fixes #3493.

Note that the method foo.each() is not de-moded, nor the other
vec routines.

This reverts commit 1be24f07.

Not quite ready.

This reverts commit 2ac64d91.

rationale: if you have an impl for &const ~[T] or &mut ~[T]
and one for &[T], the first will be more likely to work and not lead
to borrowck errors.  I could imagine us wanting to offer multiple
impls for methods we select the ptr-to-array when possible and
the slice when necessary.  Also, taking a ptr seems like fewer
adaptations than slicing, and we tend to prefer fewer adaptations.

This causes bugs since comments aren't nestable

This reverts commit 3b013cd8.