Implement basic Sapling v4 transaction parser

Details of Sapling datatypes will be filled in later; for now, they are treated
as binary blobs.

Includes code cherry-picked from upstream commit:
    7030d9eb47254499bba14f1c00abc6bf493efd91
    BIP144: Serialization, hashes, relay (sender side)

src/script/interpreter.cpp

@@ -1165,7 +1165,7 @@ uint256 SignatureHash(
         memcpy(personalization, "ZcashSigHash", 12);
         memcpy(personalization+12, &leConsensusBranchId, 4);
 
-        CBLAKE2bWriter ss(SER_GETHASH, 0, personalization);
+        CBLAKE2bWriter ss(SER_GETHASH, static_cast<int>(txTo.GetHeader()), personalization);
         // Header
         ss << txTo.GetHeader();
         // Version group ID