feat(aes-gcm-siv): re-export aes crate (#603)

added to ensure consistency with it's sister `aes-gcm` crate, which also re-exports `aes` (aes-gcm/src/lib.rs:114).

finds specific use in defining a `Key` generic for example; it takes a `KeySizeUser` which are a majority `Aes*` structs from `aes`.

aes-gcm-siv/src/lib.rs

@@ -84,6 +84,9 @@
 
 pub use aead::{self, AeadCore, AeadInPlace, Error, Key, KeyInit, KeySizeUser};
 
+#[cfg(feature = "aes")]
+pub use aes;
+
 use cipher::{
     array::Array,
     consts::{U0, U12, U16},