Replace Sync impl with Clone impl

[utkarshgupta137]

• Replace Sync impl with Clone impl: Seemed like a copy paste error, so I've removed it
• Only do ok check in Consumers when required: Couldn't find a reason that the check cannot happen later, so I've moved it inside the if block.
• Replace Sync impl with Clone impl: This is a breaking change, but it makes this crate much more ergonomic as cloning is more simpler than using Arc<MPSCProducer> or a new type.
• Currently we need to put the M part like MPSCProducer behind a shared reference like Arc, but it is internally already wrapping another Arc, so this change also removes the need for nested Arcs.
• Also, for the S part like MPSCConsumer, we don't need to use the ok AtomicBool to know if the producer is still alive, we can just use Arc::strong_count to check that.
• Arc::strong_count uses Relaxed load internally, so SCConsumer should be slightly faster in most cases.