Convert CPubKey to WitnessV0KeyHash directly

The round-tripping through PKHash has no effect, and is
potentially misleading as such.

src/script/standard.h

@@ -103,6 +103,7 @@ struct WitnessV0KeyHash : public uint160
 {
     WitnessV0KeyHash() : uint160() {}
     explicit WitnessV0KeyHash(const uint160& hash) : uint160(hash) {}
+    explicit WitnessV0KeyHash(const CPubKey& pubkey);
 };
 
 //! CTxDestination subtype to encode any future Witness version