don't allocate a vec if there are multiple adHEADmaster

It turns out that we only really iterate over those items once, in
aez_hash. We're perfectly fine just accepting an iterator, and to make
it even more generic, we use AsRef to not limit ourselves to byte
slices.

Most of this commit is just changing the function signatures of all
functions that pass a Tweak downstream.

In the next step, we could update the Aez::encrypt function to take
generic arguments. That could for example allow to pass a slice of
slices, or a vec of vecs. However, simply changing to generics is
unergonomic, as callers might have to specify generic arguments. That
sucks, so we need to figure out something else first.

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