Add support for nested list comprehension

[tohrnii]

We should allow users to define nested list comprehension like:

let transpose = [[row[i] for row in matrix] for i in 0..5]

Related issue: #132

[bobbinth]

@tohrnii, @bitwalker - is this still relevant or has it been addressed with the new structure?

[bitwalker]

We currently restrict comprehension bodies to scalar expressions, but we could easily relax that. I was under the impression we didn't want to support nested comprehensions, but there's no reason why we can't.

[bobbinth]

Yes, I think one place where this would be useful is in computing MDS matrix multiplication for RPO constraints, which would look something like this:

const MDS = [
    [7, 23, 8, 26, 13, 10, 9, 7, 6, 22, 21, 8],
    [8, 7, 23, 8, 26, 13, 10, 9, 7, 6, 22, 21],
    [21, 8, 7, 23, 8, 26, 13, 10, 9, 7, 6, 22],
    [22, 21, 8, 7, 23, 8, 26, 13, 10, 9, 7, 6],
    [6, 22, 21, 8, 7, 23, 8, 26, 13, 10, 9, 7],
    [7, 6, 22, 21, 8, 7, 23, 8, 26, 13, 10, 9],
    [9, 7, 6, 22, 21, 8, 7, 23, 8, 26, 13, 10],
    [10, 9, 7, 6, 22, 21, 8, 7, 23, 8, 26, 13],
    [13, 10, 9, 7, 6, 22, 21, 8, 7, 23, 8, 26],
    [26, 13, 10, 9, 7, 6, 22, 21, 8, 7, 23, 8],
    [8, 26, 13, 10, 9, 7, 6, 22, 21, 8, 7, 23],
    [23, 8, 26, 13, 10, 9, 7, 6, 22, 21, 8, 7]
]

fn apply_mds(state: vector[12]) -> vector[12]:
    return [sum([s * m for (s, m) in (state, mds_row)]) for mds_row in MDS]