Use signed 256-bit integer for sstore gas refund substate (#9746)

* Add signed refund

* Use signed 256-bit integer for sstore gas refund substate

* Fix tests

* Remove signed mod and use i128 directly

* Fix evm test case casting

* Fix jsontests ext signature

ethcore/src/json_tests/executive.rs

@@ -218,11 +218,11 @@ impl<'a, T: 'a, V: 'a, B: 'a> Ext for TestExt<'a, T, V, B>
 		false
 	}
 
-	fn add_sstore_refund(&mut self, value: U256) {
+	fn add_sstore_refund(&mut self, value: usize) {
 		self.ext.add_sstore_refund(value)
 	}
 
-	fn sub_sstore_refund(&mut self, value: U256) {
+	fn sub_sstore_refund(&mut self, value: usize) {
 		self.ext.sub_sstore_refund(value)
 	}
 }