ensure a Blake2 is a Writer
Revert "implement the trait on boxed ones too"

This reverts commit 8b2fb8ff85dcaf175884659be9eb0b2f99b75054.
I don't know how to ensure Box<T> is a Writer yet. TODO.
terrible hacks to align the state structs

mad props to @eddyb and @huon for helping me figure this out <3