Flat: encode UPLC arrays as lists (#7296)

* Use list encoding for flat serialisation of arrays

* Add changelog entry

Author: kwxm

plutus-core/changelog.d/20250828_234906_kenneth.mackenzie_encode_arrays_as_lists.md

@@ -0,0 +1,4 @@
+### Changed
+
+- The Flat serialisation format for UPLC built-in arrays was changed to use the encoder for lists instead of the encoder for the lazy `Vector` type.
+

plutus-core/plutus-core/src/Data/Vector/Orphans.hs

@@ -10,7 +10,15 @@ import Flat.Instances.Vector ()
 instance (Hashable a) => Hashable (Strict.Vector a) where
   hashWithSalt = Strict.foldl' hashWithSalt
 
+{- The `flat` library does not provide a `Flat` instance for
+`Vector.Strict.Vector`.  We could encode and decode strict vectors by converting
+them to and from lazy vectors (for which there is a `Flat` instance), but
+experiments show that deserialisation is actually faster by about 5-10% if we
+encode vectors as lists.  This incurs a slight size penalty (lists require one
+bit of overhead per entry whereas vectors can be encoded with an overhead of one
+byte per 255 elements), but this is offset by the decoding speedup.  Encoding
+vectors as lists also simplifies maintenance and specification. -}
 instance (Flat a) => Flat (Strict.Vector a) where
-  size = size . Strict.toLazy -- Strict to Lazy is O(1)
-  encode = encode . Strict.toLazy
-  decode = Strict.fromLazy <$> decode -- Strict from Lazy is O(1)
+  size = size . Strict.toList
+  encode = encode . Strict.toList
+  decode = Strict.fromList <$> decode