Add TaggedHash function (BIP 340)

This adds the TaggedHash function as defined by BIP340 to the hash module, which
is used in BIP340 and BIP341 to produce domain-separated hashes.

src/hash.h

@@ -15,6 +15,7 @@
 #include <uint256.h>
 #include <version.h>
 
+#include <string>
 #include <vector>
 
 typedef uint256 ChainCode;
@@ -202,4 +203,12 @@ unsigned int MurmurHash3(unsigned int nHashSeed, Span<const unsigned char> vData
 
 void BIP32Hash(const ChainCode &chainCode, unsigned int nChild, unsigned char header, const unsigned char data[32], unsigned char output[64]);
 
+/** Return a CHashWriter primed for tagged hashes (as specified in BIP 340).
+ *
+ * The returned object will have SHA256(tag) written to it twice (= 64 bytes).
+ * A tagged hash can be computed by feeding the message into this object, and
+ * then calling CHashWriter::GetSHA256().
+ */
+CHashWriter TaggedHash(const std::string& tag);
+
 #endif // BITCOIN_HASH_H