getKeysPresent is now fully lazy

.. Allowing it to be used by things in constant space!

Random statistics: git annex status has gone from taking 239 mb
of memory and 26 seconds in a repo, to 8 mb and 13 seconds.

The trick here is the unsafeInterleaveIO, and the form of the function's
recursion, which I cribbed heavily from System.IO.HVFS.Utils.recurseDirStat.
The difference is, this one goes to a limited depth and avoids statting
everything.

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