List folding is incomplete

[tohrnii]

Currently list folding doesn't work on vectors or intermediate variables representing list comprehension.

For example, the following doesn't work:

let a = [1, 2, 3]
let b = sum(a)

let a = sum([1, 2, 3])

let a = [b for b in b]
let c = sum(a)

[tohrnii]

Closed by #154