Represents coercing a value to a different type of value.
CoerceUnsized impls, storing information we
have computed about the coercion.
An overloaded autoderef step, representing a
call, with the signature
&'a T -> &'a U or
&'a mut T -> &'a mut U.
The target type is
U in both cases, with the region and mutability
being those shared by both the receiver and the returned reference.