static mpsc should be able to hand out senders after splitting

[hawkw]

it would be nice to have a

pub fn sender(&'static self) -> Option<StaticSender<T, R>>

on the StaticChannel types. The split method can only be called once because it creates the receiver, and this is a single-receiver channel, but the channel type should be able to create multiple senders, rather than just returning a StaticSender that can be Cloned.

the method would need to return an Option (or Result?) though, because it shouldn't be able to make a new sender once the channel has closed (if either the rx was dropped or all the txs have been dropped).