Use a visitor for handling -mineraddress config option

This makes it easier to add UA support.

src/init.cpp

@@ -1093,10 +1093,9 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
     if (mapArgs.count("-mineraddress")) {
         CTxDestination addr = keyIO.DecodeDestination(mapArgs["-mineraddress"]);
         if (!IsValidDestination(addr)) {
-            // Try a Sapling address
+            // Try a payment address
             auto zaddr = keyIO.DecodePaymentAddress(mapArgs["-mineraddress"]);
-            if (!IsValidPaymentAddress(zaddr) ||
-                std::get_if<libzcash::SaplingPaymentAddress>(&zaddr) == nullptr)
+            if (!std::visit(IsValidMinerAddress(), std::visit(ExtractMinerAddress(), zaddr)))
             {
                 return InitError(strprintf(
                     _("Invalid address for -mineraddress=<addr>: '%s' (must be a Sapling or transparent address)"),

src/miner.cpp

@@ -696,12 +696,10 @@ void GetMinerAddress(MinerAddress &minerAddress)
         mAddr->reserveScript = CScript() << OP_DUP << OP_HASH160 << ToByteVector(keyID) << OP_EQUALVERIFY << OP_CHECKSIG;
         minerAddress = mAddr;
     } else {
-        // Try a Sapling address
-        auto zaddr = keyIO.DecodePaymentAddress(mAddrArg);
-        if (IsValidPaymentAddress(zaddr)) {
-            if (std::get_if<libzcash::SaplingPaymentAddress>(&zaddr) != nullptr) {
-                minerAddress = std::get<libzcash::SaplingPaymentAddress>(zaddr);
-            }
+        // Try a payment address
+        auto zaddr = std::visit(ExtractMinerAddress(), keyIO.DecodePaymentAddress(mAddrArg));
+        if (std::visit(IsValidMinerAddress(), zaddr)) {
+            minerAddress = zaddr;
         }
     }
 }

src/miner.h

@@ -29,7 +29,26 @@ public:
     friend bool operator<(const InvalidMinerAddress &a, const InvalidMinerAddress &b) { return true; }
 };
 
-typedef std::variant<InvalidMinerAddress, libzcash::SaplingPaymentAddress, boost::shared_ptr<CReserveScript>> MinerAddress;
+typedef std::variant<
+    InvalidMinerAddress,
+    libzcash::SaplingPaymentAddress,
+    boost::shared_ptr<CReserveScript>> MinerAddress;
+
+class ExtractMinerAddress
+{
+public:
+    ExtractMinerAddress() {}
+
+    MinerAddress operator()(const libzcash::InvalidEncoding &invalid) const {
+        return InvalidMinerAddress();
+    }
+    MinerAddress operator()(const libzcash::SproutPaymentAddress &addr) const {
+        return InvalidMinerAddress();
+    }
+    MinerAddress operator()(const libzcash::SaplingPaymentAddress &addr) const {
+        return addr;
+    }
+};
 
 class KeepMinerAddress
 {