Start implementing the friendly front-end format

[lqd]

Implement the friendlier front-end format described in issue #26 — destined to help create isolated tests without needing rustc NLL facts, but still generating the same data structure.

Still incomplete regarding use, which might require def, and also that polonius compute liveness, to correctly emit region_live_at points. I'll open an issue for those specific points after this lands.

Here's how it looks:

// program description
universal_regions { 'a, 'b, 'c }

// block description
block B0 {
    // effects are by default emitted on the Mid point
    invalidates(L0);

    // but we can still emit some at the Start point if needed
    // by separating the Start point effects from the Mid point effects with a "/"
    invalidates(L1) / kill(L2);

    goto B1;
}
block B1 {
    use('a, 'b), outlives('a: 'b), borrow_region_at('b, L1);
}

(Sorry about opening another PR, Github prevents reopening a PR in cases of force push, which I did for the rebases)

Note: This doesn't implement the requirements for the liveness computation described here #45 (comment)