Support returning owned slices (at least backend-optionally)

[Manishearth]

Currently we support returning borrowed slices, which in non-shared-heap backends (Java, Dart, JS, Kotlin) introduce a conversion to their native heap array type.

This works great, except sometimes you just have owned data. We should support returning Box<[u8]> and friends, which converts in non-shared-heap backends, and in shared-heap backends (C++) is either unsupported, also converts, or returns a diplomat-runtime type.

Related potential question: Should diplomat support returning native Rust heap types?

Generally Diplomat tries not to provide target-language wrappers, we provide diplomat::span in C++ as a compatability thing but what we don't do is have an entire library of diplomat specific types that wrap Rust types passed over FFI. Libraries like crubit do this and they should, Diplomat's general attitude here is that if you want such a type you should make your own with exactly the APIs you need.

So, no, unless we can make it a backend attribute transform.

[Manishearth]

cc @emarteca @maurer @mgeisler This would help get rid of RustOwnedBytes in your code.

[Manishearth]

Note: when implementing this we may need to take care to ensure that returned owned vs static things are handled differently, see #798 for a similar bugfix in Dart.