Support of out-of-place decryption

[frochet]

Hello,

I'd like to have your opinion on a refactoring of the aead_gcm code to support out-of-place decryption (and aliased in-place decryption). I have a use case discussed within the following publication: https://dl.acm.org/doi/10.1145/3787927.3787929. Out-of-place decryption may offer protocols using graviola to opt for the optimization discussed within this paper (contiguous zero-copy receiver). Existing secure transports, like rustls and application using it could also potentially benefit from contiguous zero-copy.

To implement out-of-place decryption, I am thinking of minimizing code changes by:

• Introducing an enum at the 'mid' level to encapsulate the caller's intent (either in place, or out-of-place decryption). It could be something as follows:

pub enum OpenIO<'a> {
    InPlace(&'a mut [u8]),
    OutOfPlace { ciphertext_and_tag_in: &'a [u8], plaintext_out: &'a mut [u8] },
}

And then changing the decrypt's interface:

pub fn decrypt(
    &self,
    nonce: &[u8; 12],
    aad: &[u8],
    open: OpenIO<'_>,
    tag: &[u8]
) -> Result<(), Error>

And then at the 'low' level, we would have an in_ptr and out_ptr in the decrypt's interface which may or may not alias. What do you think?