* SortDirection.cs, SortContext.cs, SortSequenceContext.cs:
new infrastructure files for nested orderby/thenby calls.
* QuickSort.cs: refactored out of OrderedSequence.cs
* OrderedEnumerable.cs, OrderedSequence.cs: refactoring
to use the new SortContext infrastructure.
svn path=/trunk/mcs/; revision=102297