evm: use clean context for GetCommittedState (#383)

* keep the original context for GetCommittedState api

* fix method mutation

* keep estimateGas consistant

* added test after the original context is recovered

* add integration test for the gas consumption of sstore

* test the committed case

* move methods to keeper module

Co-authored-by: Federico Kunze Küllmer <31522760+fedekunze@users.noreply.github.com>

tests/solidity/suites/basic/contracts/Storage.sol

@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: GPL-3.0
+
+pragma solidity ^0.5.11;
+
+/**
+ * @title Storage
+ * @dev Store & retrieve value in a variable
+ */
+contract Storage {
+
+    uint256 number;
+
+    /**
+     * @dev Store value in variable
+     * @param num value to store
+     */
+    function store(uint256 num) public {
+        number = num + 1;
+        number = num;
+    }
+
+    /**
+     * @dev Return value 
+     * @return value of 'number'
+     */
+    function retrieve() public view returns (uint256){
+        return number;
+    }
+}

tests/solidity/suites/basic/test/estimateGas.js

@@ -0,0 +1,22 @@
+const Storage = artifacts.require("Storage")
+
+contract('Storage', (accounts) => {
+
+  let storage
+  beforeEach(async () => {
+    storage = await Storage.new()
+  })
+
+  it('estimated gas should match', async () => {
+      // set new value
+      let gasUsage = await storage.store.estimateGas(10);
+      expect(gasUsage.toString()).to.equal('43754');
+
+      await storage.store(10);
+
+      // set existing value
+      gasUsage = await storage.store.estimateGas(10);
+      expect(gasUsage.toString()).to.equal('28754');
+  })
+
+})