aes-gcm: Make key_size() a const fn

[ChocolateLoverRaj]

I am trying to use key_size like this:

#[derive(Debug, Serialize, Deserialize)]
struct EncryptionData {
    password_derived_key_salt: [u8; Salt::RECOMMENDED_LENGTH],
    password_derived_key: [u8; Aes256Gcm::key_size()],
}

But that isn't currently possible:

cannot call non-const fn `<AesGcm<aes_gcm::aes::Aes256, UInt<UInt<UInt<UInt<UTerm, B1>, B1>, B0>, B0>> as KeySizeUser>::key_size` in constants
calls in constants are limited to constant functions, tuple structs and tuple variants

It seems like key_size could be made const just by adding const to it.

[newpavlov]

Trait methods can not be currently made const on stable Rust. You should import the aead::generic_array::typenum::Unsigned trait and use Aes256Gcm::KeySize::USIZE.

[ChocolateLoverRaj]

I was using nightly a lot recently so I didn't realize. I will use the trait.