Add advisory for broken blake2 impls

BLAKE2b and BLAKE2s were implemented using the wrong block size. All
versions of the `blake2` crate prior to v0.8.1 compute incorrect
digests.

See: RustCrypto/MACs#19

crates/blake2/RUSTSEC-0000-0000.toml

@@ -0,0 +1,16 @@
+[advisory]
+id = "RUSTSEC-0000-0000"
+package = "blake2"
+date = "2019-08-25"
+title = "Broken implementation of BLAKE2 algorithms"
+description = """
+Implementations of BLAKE2b and BLAKE2s in `blake2` crate versions prior
+to v0.8.1 used an incorrect block size (32-bytes instead of 64-bytes for
+BLAKE2s, and 64-bytes instead of 128-bytes for BLAKE2b). This resulted in these
+implementations computing incorrect digests.
+
+The v0.8.1 release of the `blake2` crate uses the correct block sizes.
+"""
+patched_versions = [">= 0.8.1"]
+url = "https://github.com/RustCrypto/MACs/issues/19"
+category = ["crypto-failure"]