Minor performance improvements in UTxO aggregation

This makes a couple of minor improvements in the function
'aggregateUtxoCoinByCredential', as measured by the new benchmark
introduced.

I tried a few variants; see
https://drive.google.com/drive/u/1/folders/1o7r4EF0uM5gfhtf3_GoMSJdoHc_d6Xtc
for the full reports. This version seems to shave maybe 20% off of the
20K UTxO run.

Interestingly, a variant where we try to explicitly deserialise only the
stake ref (using `deserialiseAddrStakeRef`) was _considerably_ slower.
I'm not immediately sure why!

shelley/chain-and-ledger/executable-spec/src/Shelley/Spec/Ledger/EpochBoundary.hs

@@ -1,4 +1,5 @@
 {-# LANGUAGE AllowAmbiguousTypes #-}
+{-# LANGUAGE BangPatterns #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE DerivingVia #-}
@@ -105,15 +106,17 @@ deriving newtype instance
 -- | Sum up all the Coin for each staking Credential
 aggregateUtxoCoinByCredential ::
   forall era.
-  (Era era, HasField "address" (Core.TxOut era) (Addr (Crypto era))) =>
+  ( Era era,
+    HasField "address" (Core.TxOut era) (Addr (Crypto era))
+  ) =>
   Map Ptr (Credential 'Staking (Crypto era)) ->
   UTxO era ->
   Map (Credential 'Staking (Crypto era)) Coin ->
   Map (Credential 'Staking (Crypto era)) Coin
 aggregateUtxoCoinByCredential ptrs (UTxO u) initial =
-  Map.foldr accum initial u
+  Map.foldl' accum initial u
   where
-    accum out ans =
+    accum !ans out =
       case (getField @"address" out, getField @"value" out) of
         (Addr _ _ (StakeRefPtr p), c) ->
           case Map.lookup p ptrs of