Add a `unique` function that works with all kinds of iterables

The stdlib unique function can only process String and Array values and it is not possible to influence the comparison (it is always a Ruby == operation).

The unique function is really intrinsic to collections and iterators and it makes sense for it to be in Puppet instead of stdlib.

• For String and Array the function must be compatible with the stdlib version
• The function should handle all Iterables
• It should be possible to give the function a lambda that computes the value to do the equality/uniqueness check on.
• For Hash the result should be a hash with associations between the sets of keys that all have values that are computed to be equal (by the built in == or by a given lambda) and the set of those values.