Assign RUSTSEC IDs (#326)

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

crates/rio/RUSTSEC-0000-0000.toml renamed to crates/rio/RUSTSEC-2020-0021.toml

@@ -1,5 +1,5 @@
 [advisory]
-id = "RUSTSEC-0000-0000"
+id = "RUSTSEC-2020-0021"
 package = "rio"
 date = "2020-05-11"
 title = "rio allows a use-after-free buffer access when a future is leaked"