Implement support for Box<[T]> and Arc<[T]>

[nazar-pc]

Arc<[T]> is especially helpful in cases where something needs to be sent into a different thread, but cloning and double indirection are undesirable.

Right now both fail something like this:

error[E0277]: the trait bound `[subspace_core_primitives::pieces::PieceIndex]: parity_scale_codec::WrapperTypeDecode` is not satisfied
  --> crates/subspace-networking/src/protocols/request_response/handlers/cached_piece_by_index.rs:28:24
   |
28 |     pub cached_pieces: Box<[PieceIndex]>,
   |                        ^^^^^^^^^^^^^^^^^ the trait `parity_scale_codec::WrapperTypeDecode` is not implemented for `[subspace_core_primitives::pieces::PieceIndex]`, which is required by `std::boxed::Box<[subspace_core_primitives::pieces::PieceIndex]>: parity_scale_codec::Decode`
   |
   = help: the following other types implement trait `parity_scale_codec::WrapperTypeDecode`:
             std::boxed::Box<T>
             std::rc::Rc<T>
             std::sync::Arc<T>
   = note: required for `[subspace_core_primitives::pieces::PieceIndex]` to implement `parity_scale_codec::Decode`
   = note: 1 redundant requirement hidden
   = note: required for `std::boxed::Box<[subspace_core_primitives::pieces::PieceIndex]>` to implement `parity_scale_codec::Decode`

I'm not 100% sure, but suspect it is an upstream issue since downstream users will not be able to implement WrapperTypeDecode for Box<[T]> or Arc<[T]>.

[teor2345]

Specifically, this will need an implementation of WrapperTypeDecode<Wrapped = Vec<T>> for these types, because Wrapped has to be Into<Arc<[T]>>. Since slices are unsized, they can’t implement Into.

We might as well also do Rc<[T]>.

[chanderprakash20]

Hi @nazar-pc is your issue get resolved or not ?

[teor2345]

I can’t see any new PRs in the last few weeks that implement this feature. Is there something specific that you did that you think will resolve this issue?

This PR is implements the Box part of this feature, but hasn’t been reviewed or merged recently:
#565