Use uint64_t instead of size_t for serialized indices into tx.vjoinsplit

size_t has no guarantee of uniform size, and thus far has been serialized as a
uint64_t on supported platforms.

src/paymentdisclosure.h

@@ -73,7 +73,7 @@ struct PaymentDisclosurePayload {
     uint8_t version;        // 0 = experimental, 1 = first production version, etc.
     uint256 esk;            // zcash/NoteEncryption.cpp
     uint256 txid;           // primitives/transaction.h
-    size_t js;              // Index into CTransaction.vjoinsplit
+    uint64_t js;            // Index into CTransaction.vjoinsplit
     uint8_t n;              // Index into JSDescription fields of length ZC_NUM_JS_OUTPUTS
     libzcash::PaymentAddress zaddr; // zcash/Address.hpp
     std::string message;     // parameter to RPC call

src/wallet/wallet.h

@@ -159,7 +159,7 @@ public:
     // Transaction hash
     uint256 hash;
     // Index into CTransaction.vjoinsplit
-    size_t js;
+    uint64_t js;
     // Index into JSDescription fields of length ZC_NUM_JS_OUTPUTS
     uint8_t n;