Imports of package ratchet

PathSynopsis
crypto/hmacPackage hmac implements the Keyed-Hash Message Authentication Code (HMAC) as defined in U.S. Federal Information Processing Standards Publication 198.
crypto/sha256Package sha256 implements the SHA224 and SHA256 hash algorithms as defined in FIPS 180-4.
go.mau.fi/libsignal/kdfPackage kdf provides a key derivation function to calculate key output and negotiate shared secrets for curve X25519 keys.
go.mau.fi/libsignal/util/bytehelper