Ergonomic conversion of HasherNode to byte slice

[ggutoski]

In order to get a &[u8] from a HasherNode downstream users must do .as_ref().as_ref(). Example:
EspressoSystems/HotShot#2397 (comment)

The acceptable number of .as_ref() to force upon downstream users is at most 1.

[akonring]

Might be missing something but it is not clear to me why this cannot be handled in a simple manner on the Hotshot side like this: EspressoSystems/HotShot#2892

[ggutoski]

In that case, it seems there is no problem. I'm a bit surprised it works now with only one .as_ref(). I wonder whether somebody quietly slipped in a fix without noticing.