aead: add NewAead::new_varkey method

[tarcieri]

Adds a method which accepts a slice for a key, similar to the methods in the block-cipher and stream-cipher crates.

[tarcieri]

@newpavlov one thing I noticed was the method names are inconsistent among the crates.

• block-cipher uses new_varkey
• stream-cipher uses new_var

I'd suggest either standardizing on new_varkey or, to bikeshed, something like from_slice (as a breaking change across all 3 crates, for some future hypothetical release)

[burdges]

I'd suggest fn new_varkey(key: impl Borrow<[u8]>) -> Result<Self, Error> so that fixed size array work.

[newpavlov]

Stream cipher initialization function takes both key and nonce, while block cipher takes only key. For example, BlockMode also uses new_var. I guess we can rename new_varkey to new_var in the next breaking release and maybe add new_var as an alias until then.

[tarcieri]

@burdges we've used impl AsRef<[u8]> elsewhere. I think it's more exhaustive in terms of its coercions, which could be a good thing or a bad thing...

Stream cipher initialization function takes both key and nonce, while block cipher takes only key.

Ok, makes sense

[tarcieri]

For consistency with the other new_var / new_varkey APIs I'd suggest just taking a slice for now, and revisiting taking impl Borrow<[u8]> or impl AsRef<[u8]> across all three crates the next time we make a breaking change

[tarcieri]

This was released in v0.3.1:

https://docs.rs/aead/0.3.1/aead/trait.NewAead.html#method.new_varkey