Use boost::variant to represent shielded addresses and keys

libzcash::PaymentAddress has been renamed to libzcash::SproutPaymentAddress,
and a new typedef boost::variant is now libzcash::PaymentAddress. Similarly
for ViewingKey and SpendingKey.

A new class InvalidEncoding is introduced as the default boost::variant
option for each address and key type; it is used during decoding instead
of boost::optional.

All address and key storage functions in the wallet have been modified to
refer specifically to the Sprout types, as they are used very precisely.
In most other cases, the more general type is leveraged as much as possible,
and we convert to the Sprout type when necessary. This will be subsequently
wrapped in, or replaced with, context-specific functions once Sapling
types are implemented.

src/zcash/Address.cpp

@@ -6,7 +6,7 @@
 
 namespace libzcash {
 
-uint256 PaymentAddress::GetHash() const {
+uint256 SproutPaymentAddress::GetHash() const {
     CDataStream ss(SER_NETWORK, PROTOCOL_VERSION);
     ss << *this;
     return Hash(ss.begin(), ss.end());
@@ -16,24 +16,37 @@ uint256 ReceivingKey::pk_enc() const {
     return ZCNoteEncryption::generate_pubkey(*this);
 }
 
-PaymentAddress ViewingKey::address() const {
-    return PaymentAddress(a_pk, sk_enc.pk_enc());
+SproutPaymentAddress SproutViewingKey::address() const {
+    return SproutPaymentAddress(a_pk, sk_enc.pk_enc());
 }
 
-ReceivingKey SpendingKey::receiving_key() const {
+ReceivingKey SproutSpendingKey::receiving_key() const {
     return ReceivingKey(ZCNoteEncryption::generate_privkey(*this));
 }
 
-ViewingKey SpendingKey::viewing_key() const {
-    return ViewingKey(PRF_addr_a_pk(*this), receiving_key());
+SproutViewingKey SproutSpendingKey::viewing_key() const {
+    return SproutViewingKey(PRF_addr_a_pk(*this), receiving_key());
 }
 
-SpendingKey SpendingKey::random() {
-    return SpendingKey(random_uint252());
+SproutSpendingKey SproutSpendingKey::random() {
+    return SproutSpendingKey(random_uint252());
 }
 
-PaymentAddress SpendingKey::address() const {
+SproutPaymentAddress SproutSpendingKey::address() const {
     return viewing_key().address();
 }
 
 }
+
+
+bool IsValidPaymentAddress(const libzcash::PaymentAddress& zaddr) {
+    return zaddr.which() != 0;
+}
+
+bool IsValidViewingKey(const libzcash::ViewingKey& vk) {
+    return vk.which() != 0;
+}
+
+bool IsValidSpendingKey(const libzcash::SpendingKey& zkey) {
+    return zkey.which() != 0;
+}