Feature request for 'collect' function

[light-matters]

The idea here is to implement the Collect function of the wolfram language: "successively collects together terms that involve the same powers of objects matching x1, then x2, ….".

e.g.

 (collect (+ (* a x)  (* b y) (* c x) (* d y))
[x y])  

->

(+ (* (+ a c) x) (* (+ b d) y))