Add a method to strip duplicate values from already-sorted sequences.

[CTMacUser]

I'm writing a Set-like type. Each value can be in the set only once. I'm adding an initializer that will read all elements at once to perfectly build the internal binary tree in advance. Instead of worrying about removing duplicates while building the tree, it'd be better to remove all duplicates, ensuring a possibly non-decreasing sequence is a strictly-increasing one.

See pull request #257 for a sample implementation.