SortedSet: Don't maintain state in the view

SortedSet: Don't maintain state in the view

* System.Collections.Generic/SortedSet.cs (SortedSubSet.count): Remove.
(SortedSubSet..ctor): Move counting loop ...
(SortedSubSet.Count): ... here.
(SortedSubSet.TryAdd, SortedSubSet.TryRemove): Update to changes.
(SortedSubSet.GetMin, SortedSubSet.GetMax): Likewise.  Use bounds
to determine if the view is empty, rather than the count.
* Test/System.Collections.Generic/SortedSetTest.cs (ViewCount):
Test that count reflects changes in underlying set.

svn path=/trunk/mcs/; revision=155125