hasSt should lookup data in the map

Let's use the lookup function offered by Plutus insteaed of
filtering the list ourselves. Makes the code easier to read.

hydra-plutus/src/Hydra/Contract/Util.hs

@@ -20,13 +20,14 @@ hydraHeadV1 = "HydraHeadV1"
 -- 'CurrencySymbol' and 'TokenName' of 'hydraHeadV1'
 hasST :: CurrencySymbol -> Value -> Bool
 hasST headPolicyId v =
-  isJust $
-    find
-      (\(cs, tokenMap) -> cs == headPolicyId && hasHydraToken tokenMap)
-      (Map.toList $ getValue v)
+  case hasST' of
+    Just True -> True
+    _otherwise -> traceError "ST not spent in transaction"
  where
-  hasHydraToken tm =
-    isJust $ find (\(tn, q) -> q == 1 && TokenName hydraHeadV1 == tn) (Map.toList tm)
+  hasST' = do
+    tokenMap <- Map.lookup headPolicyId $ getValue v
+    quantity <- Map.lookup (TokenName hydraHeadV1) tokenMap
+    pure $ quantity == 1
 {-# INLINEABLE hasST #-}
 
 -- | Checks if all the state token (ST) for list of parties containing specific
@@ -46,4 +47,3 @@ mustNotMintOrBurn TxInfo{txInfoMint} =
   traceIfFalse "minting or burning is forbidden" $
     isZero txInfoMint
 {-# INLINEABLE mustNotMintOrBurn #-}
-