Mirror the conditions where HashSet implementation is provided to match the compilation conditions in ActivationCache.cs (added MONO)
Switch to storing a (bool)true in the HashSet to reduce object references (even if always null) that the garbage collector must walk.

Possibly should generically expose this for internal use by other HashSet-replacement needs in Ninject.
Merge pull request #58 from IDisposable/patch-2

Mirror the conditions where HashSet implementation is provided to match ...