A variant of a NonBlockingSender which can block but only for very short durations of time, much like the
channel in std or flume. Holding this variant of sender does not mean that the receiver will ever block
when receiving - the receiver provided by this crate will never block.
A SPSC sender guaranteed to never block when sending a message. This is a strong constraint, enforced
by WebAssembly on the main thread, so this should only be preferred over other mpsc channels where
non-blocking behaviour is required.