switch to the new storage API (#396)

This PR switches to the new `get_storage_or_zero` and
`set_storage_or_clear` syscalls which are more tailored towards
Solidity.

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

crates/integration/codesize.json

@@ -2,9 +2,9 @@
   "Baseline": 914,
   "Computation": 2295,
   "DivisionArithmetics": 14510,
-  "ERC20": 17499,
+  "ERC20": 17482,
   "Events": 1674,
   "FibonacciIterative": 1490,
-  "Flipper": 2102,
+  "Flipper": 2086,
   "SHA1": 8158
 }