implement self balance

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

crates/integration/contracts/Value.sol

@@ -7,6 +7,7 @@ pragma solidity ^0.8;
     "actions": [
         {
             "Instantiate": {
+                "value": 1024,
                 "code": {
                     "Solidity": {
                         "contract": "Value"
@@ -22,16 +23,30 @@ pragma solidity ^0.8;
                 "value": 123,
                 "data": "3fa4f245"
             }
+        },
+        {
+            "Call": {
+                "dest": {
+                    "Instantiated": 0
+                },
+                "data": "52da5fa0"
+            }
         }
     ]
 }
 */
 
 contract Value {
+    constructor() payable {}
+
     function value() public payable returns (uint ret) {
         ret = msg.value;
     }
 
+    function balance_self() public view returns (uint ret) {
+        ret = address(this).balance;
+    }
+
     function balance_of(address _address) public view returns (uint ret) {
         ret = _address.balance;
     }