Support transmuting `Ref<B, T>` into `Ref<B, U>`

Given `T: IntoBytes` and `U: FromBytes`, we use the same logic as `transmute_ref!`, only with `Ref` instead of references. This will need to be fallible for size and alignment reasons.

This would permit Fuchsia's Netstack3 to implement [this code](https://fuchsia-review.googlesource.com/c/fuchsia/+/1168175/2) generically, e.g. replacing `pub fn as_v1_query(&self) -> Mldv1Body<&[u8]>` with `pub fn into_v1_query(self) -> Mldv1Body`.