Add the DerefImm and DerefMut traits.
Implement DerefImm for Rc and DerefImm/DerefMut for RefCell's Ref/RefMut.
Allow overloading explicit dereferences.
auto merge of #12491 : eddyb/rust/deref, r=nikomatsakis

Add the `Deref` and `DerefMut` traits and implement overloading explicit dereferences.
Str::slice_chars() is O(end), not O(end - begin)
auto merge of #12708 : SimonSapin/rust/patch-6, r=huonw

Feel free to reject this if I’m mistaken, but the `count` in the for loop does look like it’s going from 0 to `end`.
debuginfo: Re-introduce the notion of line-table-only debuginfo.
debuginfo: Expose limited debuginfo in command line options
debuginfo: Add test case for limited debuginfo