Some tidying.
Make weak references stuff not cause GWT to choke.

Also added note about how weak references are not actually weak in JavaScript.
Someday JavaScript may support weak references in the standard (Chrome has them
as an experimental feature, I think), and we'll make this work properly then.
Add weak connections to Signal as well.

We'll see how well these work out on Signal and Value, and then roll them out
to RList, RMap and RSet if the experiment turns out to be a success.
Consolidate our type shenanigans.
Revamped the way priority is handled.

You now do: foo.connect(blah).atPriorty(N) instead of overriding priority() or
calling Slot.atPriority().

This avoids the weirdness of having priority come from a method (and therefore
conceivably changing over time, which would cause badness). It also eliminates
an issue where the priority of a collected weak cons would magically change,
thereby causing problems.
Move listener ref weakening to Connection

This means it's available on any Connection without cluttering up each
of the Reactor subclasses with *Weak versions of their connect methods.