add eraseNode tool

contracts/registry/UsernameRegistrar.sol

@@ -274,6 +274,28 @@ contract UsernameRegistrar is Controlled, ApproveAndCallFallBack {
        slashUsername(username, _reserveSecret);
    }

    /**
     * @notice Clear resolver and ownership of unowned subdomians.
     * @param _labels Sequence to erase.
     */
    function eraseNode(
        bytes32[] _labels
    ) 
        external 
    {
        uint len = _labels.length;
        require(len != 0, "Nothing to erase");
        bytes32 label = _labels[len - 1];
        bytes32 subnode = keccak256(abi.encodePacked(ensNode, label));
        require(ensRegistry.owner(subnode) == address(0), "First slash/release top level subdomain");
        ensRegistry.setSubnodeOwner(ensNode, label, address(this));
        if(len > 1) {
            eraseNodeHierarchy(len - 2, _labels, subnode);
        }
        ensRegistry.setResolver(subnode, 0);
        ensRegistry.setOwner(subnode, 0);
    }

    /**
     * @notice Migrate account to new registry, opt-in to new contract.
     * @param _label Username hash.
@@ -697,7 +719,34 @@ contract UsernameRegistrar is Controlled, ApproveAndCallFallBack {
        state = _state;
        emit RegistryState(_state);
    }
     

    /**
     * @notice recursively erase all _labels in _subnode
     * @param _idx recursive position of _labels to erase
     * @param _labels list of subnode labes
     * @param _subnode subnode being erased
     */
    function eraseNodeHierarchy(
        uint _idx,
        bytes32[] _labels,
        bytes32 _subnode
    ) 
        private 
    {
        // Take ownership of the node
        ensRegistry.setSubnodeOwner(_subnode, _labels[_idx], address(this));
        bytes32 subnode = keccak256(abi.encodePacked(_subnode, _labels[_idx]));

        // Recurse if there are more labels
        if (_idx > 0) {
            eraseNodeHierarchy(_idx - 1, _labels, subnode);
        }

        // Erase the resolver and owner records
        ensRegistry.setResolver(subnode, 0);
        ensRegistry.setOwner(subnode, 0);
    }

    /**
     * @dev Decodes abi encoded data with selector for "register(bytes32,address,bytes32,bytes32)".
     * @param _data Abi encoded data.

test/usernameregistrar.spec.js

@@ -986,6 +986,74 @@ contract('UsernameRegistrar', function () {
    });
  });
  
describe('eraseNode(bytes32[])', function() {
  it('should clear unowned subdomains of users', async () => {;
    const registrant = accountsArr[6];
    const anyone = accountsArr[5];
    await TestToken.methods.mint(registry.price).send({from: registrant});
    await TestToken.methods.approve(UsernameRegistrar.address, registry.price).send({from: registrant});  
    const username = "root";
    const usernameHash = namehash.hash(username + '.' + registry.registry);
    const label = web3Utils.sha3(username);
    const labels = [
      web3Utils.sha3("10"),
      web3Utils.sha3("9"),
      web3Utils.sha3("8"),
      web3Utils.sha3("7"),
      web3Utils.sha3("6"),
      web3Utils.sha3("5"),
      web3Utils.sha3("4"),
      web3Utils.sha3("3"),
      web3Utils.sha3("2"),
      web3Utils.sha3("1"),
      web3Utils.sha3("0"),
      web3Utils.sha3(username),
    ];
    await UsernameRegistrar.methods.register(
      label,
      utils.zeroAddress,
      utils.zeroBytes32,
      utils.zeroBytes32
    ).send({from: registrant});
    assert.equal(await ens.methods.owner(usernameHash).call(), registrant);    
    const releaseDelay = await UsernameRegistrar.methods.releaseDelay().call();
    await utils.increaseTime(releaseDelay)
    await utils.increaseTime(1000)
    await utils.increaseTime(1000)
    let subnode = usernameHash;
    for (let index = labels.length - 1; index > 0; index--) {
      const label = labels[index - 1];
      await ENSRegistry.methods.setSubnodeOwner(subnode, label, registrant).send({from: registrant}); 
      subnode = web3Utils.soliditySha3(subnode, label);
      assert.equal(await ens.methods.owner(subnode).call(), registrant);   
      
    }
   
    const resultRelease = await UsernameRegistrar.methods.release(web3Utils.sha3(username)).send({from: registrant});
    
    subnode = usernameHash;
    for (let index = labels.length - 1; index > 0; index--) {
      const label = labels[index - 1];
      subnode = web3Utils.soliditySha3(subnode, label);
      assert.equal(await ens.methods.owner(subnode).call(), registrant);   
    }

    const resultErase = await UsernameRegistrar.methods.eraseNode(
      labels
    ).send({from: anyone});
    //TODO: check events
    
    subnode = usernameHash;
    for (let index = labels.length - 1; index > 0; index--) {
      const label = labels[index - 1];
      subnode = web3Utils.soliditySha3(subnode, label);
      assert.equal(await ens.methods.owner(subnode).call(), utils.zeroAddress);   
    }
  });


});

  describe('moveRegistry(address)', function() {
    it('should move registry to new registry and migrate', async () => {
      const result = await UsernameRegistrar.methods.moveRegistry(UpdatedUsernameRegistrar.address).send();