Add very initial version of smart contracts with tests

contracts/Miksi.sol

@@ -0,0 +1,52 @@
+pragma solidity ^0.6.0;
+
+import './verifier.sol';
+
+contract Miksi {
+  Verifier    verifier;
+
+  constructor( address _verifierContractAddr) public {
+    verifier = Verifier(_verifierContractAddr);
+  }
+
+  mapping(uint256 => Deposit) deposits;
+
+  struct Deposit {
+    uint256 coinCode;
+    uint256 amount;
+  }
+
+  function deposit(
+            uint256 coinCode,
+            uint256 amount,
+            uint256 commitment
+  ) public {
+    deposits[commitment] = Deposit(coinCode, amount);
+  }
+
+  function getDeposit(
+            uint256 commitment
+  ) public view returns (uint256, uint256) {
+    return (
+      deposits[commitment].coinCode,
+      deposits[commitment].amount
+    );
+  }
+
+  function withdraw(
+            uint256 commitment,
+            uint[2] memory a,
+            uint[2][2] memory b,
+            uint[2] memory c
+  ) public {
+
+    uint256[3] memory input = [
+      deposits[commitment].coinCode,
+      deposits[commitment].amount,
+      commitment
+    ];
+    require(verifier.verifyProof(a, b, c, input), "zkProof withdraw could not be verified");
+
+    // zk verification passed, proceed with the withdraw
+  }
+}