Call ImmutableList<T>.AddRange from ConvertAll and FindAll

Replace any remaining calls to ImmutableList<T>+Node.Add and
ImmutableList<T>+Builder.Add from within a loop to calls to the
equivalent AddRange method
Add back before implementation of ImmutableList<T>.AddRange for
performance testing
Add performance test console app
Add benchmarks for ImmutableList<T>.FindAll and .ConvertAll