This change adds a new variant of "addObserver:forKeyPath:options:context:"
that takes a "KVOContext" instead of an unsafe void*. The variant
delegates to the original method, but first 'retain's the object
before turning it into an unsafe pointer. The API is then matched
with a variant of 'removeObserver:forKeyPath:context:' which
delegates to the original method and then 'release's it.
This vision here is that Swift clients of this API will use this
variant, and not the unsafe one. A refinement (later) is to
not expose the original methods at all, and provide a new method
'observeValueForKeyPath:ofObject:change:kvoContext:' that implements
a thunk that delegates to 'observeValueForKeyPath:ofObject:change:context:'
and does the void* cast (which the code in the test case does).
This needs to go through API review; names are strawman names.
Swift SVN r17325
