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/Note.cpp

@@ -34,7 +34,7 @@ uint256 SproutNote::cm() const {
     return result;
 }
 
-uint256 SproutNote::nullifier(const SpendingKey& a_sk) const {
+uint256 SproutNote::nullifier(const SproutSpendingKey& a_sk) const {
     return PRF_nf(a_sk, rho);
 }
 
@@ -46,7 +46,7 @@ SproutNotePlaintext::SproutNotePlaintext(
     r = note.r;
 }
 
-SproutNote SproutNotePlaintext::note(const PaymentAddress& addr) const
+SproutNote SproutNotePlaintext::note(const SproutPaymentAddress& addr) const
 {
     return SproutNote(addr.a_pk, value_, rho, r);
 }