Prefer `&mut [u8]` over returning a newly allocated `Vec<u8>`; derive the latter from the former

[WildCryptoFox]

#[no_std] is already common between these crates but when they pull in alloc for Vec, and always allocate, you lose the ability to reuse allocations between calls.

At least an _in_place.

[newpavlov]

Are you talking about the aead crate? We had a very lengthy discussion about non-allocating API. So we decided to start with a simple non-controversial allocating API and add non-allocating methods later.

[WildCryptoFox]

@newpavlov I am, though I didn't want to discriminate against other cases where it may apply too. Thanks for the link. Later is fine.

[tarcieri]

I've been meaning to submit a follow-up PR with my thoughts as I'd like to address the in-place API before moving Miscreant over to the traits in the aead crate

[tarcieri]

We added in-place APIs to the aead crate in #58 and #59