Add support for spending keys to the basic key store

src/zcash/Address.cpp

@@ -8,7 +8,7 @@ uint256 ViewingKey::pk_enc() {
     return ZCNoteEncryption::generate_pubkey(*this);
 }
 
-ViewingKey SpendingKey::viewing_key() {
+ViewingKey SpendingKey::viewing_key() const {
     return ViewingKey(ZCNoteEncryption::generate_privkey(*this));
 }
 
@@ -16,8 +16,8 @@ SpendingKey SpendingKey::random() {
     return SpendingKey(random_uint252());
 }
 
-PaymentAddress SpendingKey::address() {
+PaymentAddress SpendingKey::address() const {
     return PaymentAddress(PRF_addr_a_pk(*this), viewing_key().pk_enc());
 }
 
-}
+}