Use solidity mapper to get value for mapping and nested mapping (balance and allowance) (#48)

* Implement getting value for basic mapping type.

* Add test for basic mapping type.

* Implement getting value for nested mapping type.

Co-authored-by: nikugogoi <95nikass@gmail.com>

packages/solidity-mapper/test/contracts/TestMappingTypes.sol

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.7.0;
+
+contract TestMappingTypes {
+    // Mapping type variable occupies one single slot but the actual elements are stored at a different storage slot that is computed using a Keccak-256 hash.
+    // https://docs.soliditylang.org/en/v0.8.4/internals/layout_in_storage.html#mappings-and-dynamic-arrays
+    mapping (address => mapping (address => uint)) private nestedAddressUintMap;
+
+    // Mapping type variable occupies the next single slot.
+    mapping(address => uint) public addressUintMap;
+
+    // Set variable addressUintMap.
+    function setAddressUintMap(uint value) external {
+        addressUintMap[msg.sender] = value;
+    }
+
+    // Set variable nestedAddressUintMap.
+    function setNestedAddressUintMap(address addressValue, uint uintValue) external {
+        nestedAddressUintMap[msg.sender][addressValue] = uintValue;
+    }
+}