Primary changes:

• SortedSet's ctor has a loop that's O(D * N), where N is the number of elements being added to the set and D is the number of duplicates in those elements. As the number of duplicates D grows, the time it takes to construct a sorted set grows polynomially, approaching O(N^2), which is particularly bad given that the purpose of a set is to help remove duplicates. The core cause is a loop over the elements to determine whether element n is equal to n-1; if it is, element n is removed, but it's removed via a call to List.RemoveAt, which will shift down all of the elements above it in the list. A better implementation can make this O(N) instead; the ctor overall is still then O(N log N), due to the sort, but that's better than O(N^2).
• To sort the elements, the ctor builds a List of the elements but then uses ToArray to get an array used to actually build the set; this incurs an unnecessary (and potentially large) array allocation and copy. We can avoid that entirely by just building up the array manually using the same logic that List does. That same logic is actually duplicated in multiple places, including in Stack, so I've extracted it as a helper to use in both places (there are a few other places in corefx that use the same logic, hence I put the helper into a Common file, but I did not condense those other uses as part of this commit). This then further helps with Stack perf in a few corner cases, such as when a Stack is constructed with an empty source. (It's not clear to me why Queue doesn't have the same IColection-related logic; probably just an oversight, but I didn't want to change its behavior with the additional ICollection check.)
• SortedSet's BreadthFirstTreeWalk (which is used by several members, such as RemoveWhere) is currently using a List as a queue, doing a RemoveAt(0) to dequeue, which will incur the cost of shifting all elements down. Using a Queue is both cleaner and better performing.
• As I was touching Stack and Queue, I noticed that they had an empty array static field, which was just being initialized to Array.Empty(). The field is unnecessary, and the call site can just use Array.Empty directly.

Fixes #1953.