De-monomorphize spsc consumer, producer and iterators

This needs a workaround for the `const` use case however

Author: sosthene-nitrokey

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - `bytes::BufMut` is now implemented on `VecInner`.
 - Removed generic from `history_buf::OldestOrdered`.
+- Removed generic from `spsc::Consumer`, `spsc::Producer` and `spsc::Iter`.
 
 ### Fixed