Support arbitrary properties for a user-supplied sched_algorithm implementation.

Introduce fiber_properties base class + sched_algorithm_with_properties<PROPS>
template base class. Introduce set_properties(), get_properties methods on
worker_fiber; introduce fm_properties() functions for fiber_manager; introduce
properties() methods on fiber, this_fiber and sched_algorithm_with_properties.

Introduce sched_algorithm::property_change() method to support changes to
properties that require immediate notification to the relevant sched_algorithm
implementation -- such as a change in priority of a fiber already in that
sched_algorithm's ready queue.

Juggle namespaces so it's possible to implement a sched_algorithm subclass
using only fiber_base* (now promoted to boost::fibers namespace) rather than
having to touch detail::worker_fiber*. Hoist nxt_ pointer from worker_fiber
into fiber_base: we have many different needs (fortunately mutually exclusive)
to put a fiber_base subclass on an intrusive linked list.

In the migration example, introduce 'affinity' subclass of fiber_properties
containing 'thread_affinity' data member. Reimplement workstealing_round_robin
as a sched_algorithm_with_properties<affinity> subclass; tweak its steal()
method to honor thread_affinity.

Remove thread_affinity as a flag for every fiber, and the surprising number of
support functions. Similarly, remove priority and its corresponding support
functions. We could and probably should introduce an example like the
migration example illustrating how one might implement a priority scheme, but
I haven't yet tackled that.

Simplify certain queue implementations using pointer-to-pointer tactics for
the tail pointer and/or the scan pointer.

(Please skim lightly over the .qbk edits: they are incomplete, and by now
somewhat out-of-date. They got included only because the commit was still
pending at the time I created the pull request.)