storage keys and values should be big endian (#277)

Storage keys and values are big endian. Keeping them LE was a pre-mature
optimization because for the contract itself it this is a no-op and thus
not observable. However we should consider the storage layout as part of
the contract ABI. The endianness of transient storage values are still
kept as-is.

---------

Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>

crates/integration/codesize.json

@@ -2,9 +2,9 @@
   "Baseline": 1443,
   "Computation": 2788,
   "DivisionArithmetics": 9748,
-  "ERC20": 19200,
+  "ERC20": 19150,
   "Events": 2201,
   "FibonacciIterative": 2041,
-  "Flipper": 2632,
-  "SHA1": 8958
+  "Flipper": 2691,
+  "SHA1": 8997
 }