tests/RingBuffer: Remove test against private data.

elements will be made private, so this test will never compile.
KVO: add Block type for KVO closures.

Will be moving it out of the internal KeyValueObservationForwarder
class since the Block type needs to be public.
KVO: Swap all use of KeyValueObserverBlock with the Block type.
KVO: remove use of (Array)?Forwarder enum case.

No point to it- just use Object/ArrayObject.
KVO: remove Forwarder/ArrayForwarder.

Unused.
KVO: Remove NsKeyValueObservingOptions.combined

Unused.
KVO: remove KeyValueObservationForwarder.KeyValueObserverBlock

Unused.
notifObserver: reorder observeNotification arguments.

Makes a little more sense to have sentBy before onQueue. Nevermind that
the argument order here is dictated by what gets the compiler to ignore
unwanted arguments without erroring out because the block argument
gets passed as one of the optionals instead of as the actual last
argument.
Give everything access qualifiers.

Everything that isn't marked internal is currently public aside from a
few private members of RingBuffer<T>, because you really don't need to
be touching the read/write heads or the buffer's internal array.

There just isn't a lot of stuff to make private right now.

---- marked `internal` notes ----
- *Observers' disconnect methods are all internal (use the appropriate
  disconnect function).

- KeyValueObservationForwarder is internal. I'd prefer to restrict it
  even further to just its translation unit, but I don't think Swift
  will ever get a C-like static.

- NSKeyValueObservingOptions extension made internal. You get to pass
  an array of options for KVO functions, so I don't see a problem with
  hiding something I'm just using internally.

- QBarrierUnsupportedException made internal -- may be removed entirely
  since I think having the barrier functions on WorkQueue is probably
  a bad idea since barrier-like behavior for NSOperationQueue requires
  work that I'd rather not spend on it (i.e., I'd rather just use a
  dispatch queue if barriers are necessary).