Merge #15512: Add ChaCha20 encryption option (XOR)

2dfe27517 Add ChaCha20 bench (Jonas Schnelli)
2bc2b8b49 Add ChaCha20 encryption option (XOR) (Jonas Schnelli)

Pull request description:

  The current ChaCha20 implementation does not support message encryption (it can only output the keystream which is sufficient for the RNG).

  This PR adds the actual XORing of the `plaintext` with the `keystream` in order to return the desired `ciphertext`.

  Required for v2 message transport protocol.

ACKs for commit 2dfe27:
  jnewbery:
    Looks good. utACK 2dfe275171.
  jnewbery:
    utACK 2dfe275171
  sipa:
    utACK 2dfe275171
  ryanofsky:
    utACK 2dfe275171. Changes since last review are just renaming the Crypt method, adding comments, and simplifying the benchmark.

Tree-SHA512: 84bb234da2ca9fdc44bc29a786d9dd215520f81245270c1aef801ef66b6091b7793e2eb38ad6dbb084925245065c5dce9e5582f2d0fa220ab3e182d43412d5b5

src/Makefile.bench.include

@@ -21,6 +21,7 @@ bench_bench_bitcoin_SOURCES = \
   bench/duplicate_inputs.cpp \
   bench/examples.cpp \
   bench/rollingbloom.cpp \
+  bench/chacha20.cpp \
   bench/crypto_hash.cpp \
   bench/ccoins_caching.cpp \
   bench/gcs_filter.cpp \