Prevent storing and passing DST values

[nrc]

We must forbid storing or passing DST values. I.e., DST values may not be l-values.

The following should be illegal:

fn g<type X>(x: X) {...}
fn f(x: ~[T]) {
    let y = *x; //~ERROR lvalue is dynamically sized
    g(*x); //~ERROR lvalue is dynamically sized
}

I think we can enforce this by preventing storage locations having DST, rather than checking de-references.

Note that pointers to DST values are themselves sized, so may be lvalues. As a follow-up, we may allow DST values to be passed as arguments, but that will involve some pretty gross hacks.

We already prevent DST values being stored in fields.

[nrc]

assigned to me;
cc @nikomatsakis