Add rust bindings to AES-GCM-SIV through the EVP_AEAD_* API's

Implemented a generic Aead trait and struct against the EVP_AEAD
API's, which can be used to provide bindings to all of the AEAD's
provided by boringssl. Starting with AES_GCM_SIV, but will expand
to more AEAD's.

Change-Id: I7d4113f3d49ff40de3ccb76424f9a25d25797e82
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/59965
Reviewed-by: Bob Beck <bbe@google.com>
Commit-Queue: Bob Beck <bbe@google.com>
diff --git a/rust/bssl-crypto/src/lib.rs b/rust/bssl-crypto/src/lib.rs
index f4d1291..c538467 100644
--- a/rust/bssl-crypto/src/lib.rs
+++ b/rust/bssl-crypto/src/lib.rs
@@ -26,6 +26,9 @@
 
 extern crate core;
 
+/// Authenticated Encryption with Additional Data algorithms.
+pub mod aead;
+
 /// AES block operations.
 pub mod aes;
 
@@ -44,7 +47,7 @@
 /// Random number generation.
 pub mod rand;
 
-/// BoringSSL implemented memory-manipulation operations.
+/// Memory-manipulation operations.
 pub mod mem;
 
 #[cfg(test)]