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

[joshlf]

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 generically, e.g. replacing pub fn as_v1_query(&self) -> Mldv1Body<&[u8]> with pub fn into_v1_query(self) -> Mldv1Body<B>.