Add Cypher Stack's audit of /crypto

The current /crypto folder, as of this commit, is identical, except for the
years in the copyright statements. GitHub's CI also passed for the previous
commit, ensuring the repo's integrity during that commit. This now establishes
trust for /crypto, which will be used to update documentation and create
releases.

audits/Cypher Stack crypto March 2023/LICENSE

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+MIT License
+
+Copyright (c) 2023 Cypher Stack
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

audits/Cypher Stack crypto March 2023/README.md

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+# Cypher Stack /crypto Audit, March 2023
+
+This audit was over the /crypto folder, excluding the ed448 crate, the `Ed448`
+ciphersuite in the ciphersuite crate, and the `dleq/experimental` feature. It is
+encompassing up to commit 669d2dbffc1dafb82a09d9419ea182667115df06.
+
+Please see https://github.com/cypherstack/serai-audit for provenance.