CanonicalSerde for Rc and Arc

[alxiong]

1. Currently only CanonicalSerialize is implemented for Rc<T> here, but the deserialize counterpart is missing.
2. Maybe consider relaxing the feature flag for CanonicalSerde for Arc<T> which is currently only available in feature="std", so I'm hoping:
   • (conditionally) re-export alloc::sync under #[cfg(target_has_atomic = "ptr")] in ark-std as alloc does it.
   • change the feature directive inside ark-serialize to reflect the change.
   • (these changes make it possible to use Arc on platforms that supports atomic ops and pointers, which include MIPS but unfortunately not WASM I believe)

The second request is optional, subject to maintainer's judgement on how relevant or useful will it be to add. But the first point is easy and nice to have.

[burdges]

ark-std lags std's alloc in a few ways, like arkworks-rs/std#44

If rust stabilizes error_in_core then ark-std could largely be deprecated. If someone wants something within the next few years then they should probably update parts of ark-std to a more recent alloc. It's unlikely to be contentious to make ark-std more like alloc.

[burdges]

We might've ark-serialize bugs similar to paritytech/parity-scale-codec#426 I suppose serde could've worse similar bugs.