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/paymentdisclosure.h

@@ -38,12 +38,12 @@ struct PaymentDisclosureInfo {
     uint256 joinSplitPrivKey; // primitives/transaction.h
     // ed25519 - not tied to implementation e.g. libsodium, see ed25519 rfc
 
-    libzcash::PaymentAddress zaddr;
+    libzcash::SproutPaymentAddress zaddr;
 
     PaymentDisclosureInfo() : version(PAYMENT_DISCLOSURE_VERSION_EXPERIMENTAL) {
     }
 
-    PaymentDisclosureInfo(uint8_t v, uint256 esk, uint256 key, libzcash::PaymentAddress zaddr) : version(v), esk(esk), joinSplitPrivKey(key), zaddr(zaddr) { }
+    PaymentDisclosureInfo(uint8_t v, uint256 esk, uint256 key, libzcash::SproutPaymentAddress zaddr) : version(v), esk(esk), joinSplitPrivKey(key), zaddr(zaddr) { }
 
     ADD_SERIALIZE_METHODS;
 
@@ -75,7 +75,7 @@ struct PaymentDisclosurePayload {
     uint256 txid;           // primitives/transaction.h
     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
+    libzcash::SproutPaymentAddress zaddr; // zcash/Address.hpp
     std::string message;     // parameter to RPC call
 
     ADD_SERIALIZE_METHODS;