consider removing Hpke's internal RwLock

[japaric]

hpke_rs::Hpke wraps its internal PRNG in a RwLock. I assume this to make its methods work with just access to a shared reference (&self) and to keep the type Send and Sync.

an alternative is to remove the internal RwLock and make Hpke's methods require a mutable reference. the end user then can wrap the whole Hpke object in a Arc<RwLock>, or Arc<Mutex>, or RefCell (if they don't need Send / Sync-ness) depending on their application requirements so it makes the API more flexible.

the internal RwLock prevents the crate from being no-std compatible so removing it clears the path towards no-std support

[franziskuskiefer]

Yes, this was done for the evercrypt provider. But I agree that it would be better to drop it. Using a mutable reference sounds like the way to go.

[japaric]

done in PR #55