Macro aead::impl_decrypt_in_place

macro_rules! impl_decrypt_in_place {
    ($aead : expr, $nonce : expr, $aad : expr, $buffer : expr) => { ... };
}

Implement the decrypt_in_place method on AeadInPlace and [AeadMutInPlace], using a macro to gloss over the &self vs &mut self.

Assumes a postfix authentication tag. AEAD ciphers which do not use a postfix authentication tag will need to define their own implementation.