code clean-up: @discardableResult, unused params, simplify getting specific relays from pool

Closes: #635

damus/Nostr/ProofOfWork.swift

@@ -72,7 +72,7 @@ func char_to_hex(_ c: UInt8) -> UInt8?
     return nil;
 }
 
-
+@discardableResult
 func hex_decode(_ str: String) -> [UInt8]?
 {
     if str.count == 0 {

damus/Nostr/RelayPool.swift

@@ -195,27 +195,13 @@ class RelayPool {
             relay.connection.send(req)
         }
     }
-
+    
     func get_relays(_ ids: [String]) -> [Relay] {
-        var relays: [Relay] = []
-
-        for id in ids {
-            if let relay = get_relay(id) {
-                relays.append(relay)
-            }
-        }
-
-        return relays
+        relays.filter { ids.contains($0.id) }
     }
-
+    
     func get_relay(_ id: String) -> Relay? {
-        for relay in relays {
-            if relay.id == id {
-                return relay
-            }
-        }
-
-        return nil
+        relays.first(where: { $0.id == id })
     }
     
     func record_last_pong(relay_id: String, event: NostrConnectionEvent) {