[Cs3] updating set/multiset elements

[Mikhail Nesterenko]

CS3 students,

This is the question I asked myself to double-check during class. So,
for sets and multisets, they key and value are the same. This means
that key is constant and cannot (or, rather, should not) be updated as
it destroys internally implemented balanced tree. I updated slides to
reflect that.

Thanks,
-- 
Mikhail