Handle recursive field drop and user-defined destructors

[solson]

It would be best to take advantage of the ElaborateDrops MIR pass, which is currently run after the after_analysis callback that Miri uses.

I'm currently looking into adding a callback between after_analysis and after_trans for this purpose.

[solson]

We already have ElaborateDrops in place, but this feature would likely be a lot easier to implement if rustc generated actual Mir for drop glue.

[solson]

@oli-obk Is enough of this done that we can close this? (due to #74)

[oli-obk]

Nope, we still have some things not working (like Option<&T>, which is a Layout::RawNullablePointer. (in fact the tests for the impl for that just finished successfully)

I'm running the rustc run-pass tests now to see if any other cases pop up.

Also I don't think the following works:

struct Foo;
impl Drop for Foo { .. }

enum Bar {
    A(Box<i32>),
    B(Foo),
}