Paper over privacy issues with Deref by changing field names.

Paper over privacy issues with Deref by changing field names.

Types that implement Deref can cause weird error messages due to their
private fields conflicting with a field of the type they deref to, e.g.,
previously

   struct Foo { x: int }

   let a: Arc<Foo> = ...;
   println!("{}", a.x);

would complain the the `x` field of `Arc` was private (since Arc has a
private field called `x`) rather than just ignoring it.

This patch doesn't fix that issue, but does mean one would have to write
`a._ptr` to hit the same error message, which seems far less
common. (This patch `_`-prefixes all private fields of
`Deref`-implementing types.)

cc #12808