Derive failure for `&[T]`

[Bowen Wang]

If I have the following structs

#[derive(BorshSerialize]
struct A {}
#[derive(BorshSerialize)]
struct B<'a> {
  a: &'a [A]
}

The derive for B will fail even though we have derive for [T] and &T if T implements BorshSerialize. It fails with "size for value cannot be known at compile time. If I change [A] to Vec<A>, it works.

[Evgeny Kuzyakov]

I think BorshSerialize requires Sized, but we probably can lift this requirement. You still need it for BorshDeserialize