This does not yet handle the case of mixed deref patterns with normal
constructors; it'll ICE in `Constructor::is_covered_by`. That'll be
fixed in a later commit.
This allows deref patterns to move out of boxes.
Implementation-wise, I've opted to put the information of whether a
deref pattern uses a built-in deref or a method call in the THIR. It'd
be a bit less code to check `.is_box()` everywhere, but I think this way
feels more robust (and we don't have a `mutability` field in the THIR
that we ignore when the smart pointer's a box). I'm not sure about the
naming (or using `ByRef`), though.
Since deref patterns on boxes will be lowered differently, I'll be
making a separate test file for them. This makes sure we're still
testing the generic `Deref(Mut)::deref(_mut)`-based lowering.
