Implement ChaCha20 Crypto in stdlib (#2643)

* ChaCha20 implementation, first pass

* fix bug with clone_slice when length is 0

* final ChaCha20 crypto tidying

* final adjustments; add benchmark

* add guards everywhere else or w/e

* stdlib 'i++' conformity

* release notes & security warning updates

* update tests; cleanup; default counter should be 0 not 1

* remove prints in test file

* add extra unit tests for unaligned buffers

Co-authored-by: Manu Linares <mbarriolinares@gmail.com>

* one final alignment test

* nice contraction of tests w/ some paranoia sprinkled in

* nearly double the efficiency of chacha20's transform

Co-authored-by: Manu Linares <mbarriolinares@gmail.com>

* fix memory leak in test case

* improve one of the unit tests to cover more cases

* greatly simplify chacha20 'transform'

Co-authored-by: Manu Linares <mbarriolinares@gmail.com>

---------

Co-authored-by: Manu Linares <mbarriolinares@gmail.com>

benchmarks/stdlib/crypto/chacha20.c3

@@ -0,0 +1,25 @@
+// Copyright (c) 2025 Zack Puhl <github@xmit.xyz>. All rights reserved.
+// Use of this source code is governed by the MIT license
+// a copy of which can be found in the LICENSE_STDLIB file.
+module chacha20_benchmarks;
+import std::crypto::chacha20;
+
+
+fn void initialize_bench() @init
+{
+	set_benchmark_warmup_iterations(3);
+	set_benchmark_max_iterations(1024);
+}
+
+const char[] KEY = x'98bef1469be7269837a45bfbc92a5a6ac762507cf96443bf33b96b1bd4c6f8f6';
+const char[] NONCE = x'44e792d63335abb1582e9253';
+const uint COUNTER = 42;
+
+char[] one_megabyte = { [0..1024*1024] = 0xA5 };
+
+// This doesn't test both encryption + decryption, because it's a symmetric operation that shares
+//   a single common data transformation. Testing one limb is enough.
+fn void gogo_chacha20() @benchmark
+{
+	chacha20::encrypt_mut(one_megabyte[..], KEY, NONCE, COUNTER);
+}