Auto merge of #3206 - bitcartel:3194_polymorphic_note_class, r=str4d

Update note classes to be polymorphic

Closes #3194.  Lays foundation for introduction of Sapling notes through refactoring and creation of a class hierarchy for Sprout notes.  This PR updates some tests, but otherwise is a no-op.

src/test/coins_tests.cpp

@@ -204,7 +204,7 @@ uint256 appendRandomCommitment(ZCIncrementalMerkleTree &tree)
     libzcash::SpendingKey k = libzcash::SpendingKey::random();
     libzcash::PaymentAddress addr = k.address();
 
-    libzcash::Note note(addr.a_pk, 0, uint256(), uint256());
+    libzcash::SproutNote note(addr.a_pk, 0, uint256(), uint256());
 
     auto cm = note.cm();
     tree.append(cm);