libsync: Add safer abstraction for SPSC queue.

The current spsc implementation doesn't enforce single-producer
single-consumer usage and also allows unsafe memory use through
peek & pop.

For safer usage, `new` now returns a pair of owned objects which
only allow consumer or producer behaviours through an `Arc`.
Through restricting the mutability of the receiver to `mut` the
peek and pop behaviour becomes safe again, with the compiler
complaining about usage which could lead to problems.

To fix code broken from this, update:
Queue::new(x) -> unsafe { Queue::unchecked_new(x) }

[breaking-change]

For an example of broken behaviour, check the added test which uses the unchecked constructor.