Allow user to specify aliasing model for Miri

[GoldsteinE]

Very useful for comparing stacked borrows and tree borrows.

An example for testing:

fn main() {
    let val = [1u8, 2];
    let ptr = &val[0] as *const u8;
    let _val = unsafe { *ptr.add(1) };
}

is valid under Tree Borrows, but not under Stacked Borrows.

UI looks like this:

[GoldsteinE]

The one problem I have with my implementation is that this knob is not saved when sharing via link. I could easily fix this, but it will add an extra &aliasing_model=stacked at the end of every single URL. Alternatively, I could make “Share” omit unchanged parameters, but I suppose there’s a reason why it isn’t this way already.

[GoldsteinE]

@shepmaster Hi! Looking at the commit history, I think I should ask you to review these changes.