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: