core/crypto: Add chacha20

This package implements the ChaCha20 stream cipher as specified in
RFC 8439, and the somewhat non-standard XChaCha20 variant that supports
a 192-bit nonce.

While an IETF draft for XChaCha20 standardization exists,
implementations that pre-date the draft use a 64-bit counter, instead of
the IETF-style 32-bit one.  This implementation opts for the latter as
compatibility with libsodium is more important than compatibility with
an expired IETF draft.

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