refactor: Use uint64_t over size_t for serialized-size values

The values are small enough to fit in size_t, but to avoid having to
think about it, just use uint64_t consistently for all architectures.

On 64-bit systems, this refactor is a no-op. On 32-bit systems, it could
avoid bugs in the theoretical and unexpected case where a 32-bit size_t
is too small and overflows.

src/wallet/spend.cpp

@@ -1034,7 +1034,7 @@ void DiscourageFeeSniping(CMutableTransaction& tx, FastRandomContext& rng_fast,
     }
 }
 
-size_t GetSerializeSizeForRecipient(const CRecipient& recipient)
+uint64_t GetSerializeSizeForRecipient(const CRecipient& recipient)
 {
     return ::GetSerializeSize(CTxOut(recipient.nAmount, GetScriptForDestination(recipient.dest)));
 }