Add some f*cking features to `set`

[danilopedraza]

Sets do nothing. I need several features:

• [ ] Composing with another iterables: { k : k in [1, 2, 2] } = { 1, 2 }
• [ ] Use them as properties? I'm not convinced of this anymore: { k : k % 2 == 0 && k % 3 == 0 }
• [ ] Join them with the + operator
• [ ] Intersect them with the * operator
• [ ] Get the difference with the - operator
• [ ] Add an element with the cons syntax: {1|{2}} = {1|[1,2]} = {1, 2}