This continues 2c8063f4 (*: Channels must be compared by ==, not by "is"
even for nilchan) and changes comparision of .done() for different
contexts from "is" to ==.

The reason is that soon we are going to move contexts into C, and this
way in context tree there potentially can be PyCtx -> CCtx -> PyCtx
chains, and while all those contexts could have the same C-level channel,
it would be difficult to propagate the knowledge that Python-level
pychan wrapper should be also exactly the same.

We already treat channel objects as pointers and require to compare them
by ==, so there should be no loss here. However to keep performance of
repeated

	select(
	    ctx.done(),
	    ...
	)

we continue to require that .done() for a particular python context
object returns exactly the same channel object every time.