Refactoring: rename NotePlaintext --> SproutNotePlaintext

Also renames:
CNotePlaintextEntry --> CSproutNotePlaintextEntry
CUnspentNotePlaintextEntry --> CUnspentSproutNotePlaintextEntry

src/zcash/JoinSplit.cpp

@@ -246,7 +246,7 @@ public:
             ZCNoteEncryption encryptor(h_sig);
 
             for (size_t i = 0; i < NumOutputs; i++) {
-                NotePlaintext pt(out_notes[i], outputs[i].memo);
+                SproutNotePlaintext pt(out_notes[i], outputs[i].memo);
 
                 out_ciphertexts[i] = pt.encrypt(encryptor, outputs[i].addr.pk_enc);
             }