Implement distinct & distinctUntil Operators

[njlr]

// pseudo-code
xs = [ 1, 1, 2, 3, 1, 2, 2, 3 ]
distinct(xs) = [ 1, 2, 3 ]
distinctUntil(xs) = [ 1, 2, 3, 1, 2, 3 ]

Open to suggestions on names.