VectorKeyEnumeratorClass now implements IDisposable
ArrayKeyEnumeratorClass now implements IDisposable
ShimEnumerator now implements IDisposable
ArrayEnumeratorClass now implements IDisposable
VectorEnumeratorClass now implements IDisposable
Mark interface regions in pscorlib/Dictionary.cs
Merge branch 'fix-warning-about-idisposable'

While inlining methods I noticed some classes did not implement the
IDisposable interface, even though they had a Dispose method. This fixes
the warning by implementing such interface.