ETH bonding contract

The contract is used for ETH only bonding. It defined withdraw function
that is different from the one defined in KeepBonding contract as there
is no grantee role. Other parts of the code are common for token bonding
and eth bonding.

Author: nkuba

solidity/contracts/ETHBonding.sol

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+/**
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+  ▓▓▓▓▓▓    ▓▓▓▓▓▓▓▀    ▐▓▓▓▓▓▓    ▐▓▓▓▓▓   ▓▓▓▓▓▓     ▓▓▓▓▓   ▐▓▓▓▓▓▌   ▐▓▓▓▓▓▓
+  ▓▓▓▓▓▓▄▄▓▓▓▓▓▓▓▀      ▐▓▓▓▓▓▓▄▄▄▄         ▓▓▓▓▓▓▄▄▄▄         ▐▓▓▓▓▓▌   ▐▓▓▓▓▓▓
+  ▓▓▓▓▓▓▓▓▓▓▓▓▓▀        ▐▓▓▓▓▓▓▓▓▓▓         ▓▓▓▓▓▓▓▓▓▓▌        ▐▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓
+  ▓▓▓▓▓▓▀▀▓▓▓▓▓▓▄       ▐▓▓▓▓▓▓▀▀▀▀         ▓▓▓▓▓▓▀▀▀▀         ▐▓▓▓▓▓▓▓▓▓▓▓▓▓▓▀
+  ▓▓▓▓▓▓   ▀▓▓▓▓▓▓▄     ▐▓▓▓▓▓▓     ▓▓▓▓▓   ▓▓▓▓▓▓     ▓▓▓▓▓   ▐▓▓▓▓▓▌
+▓▓▓▓▓▓▓▓▓▓ █▓▓▓▓▓▓▓▓▓ ▐▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓ ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓  ▓▓▓▓▓▓▓▓▓▓
+▓▓▓▓▓▓▓▓▓▓ ▓▓▓▓▓▓▓▓▓▓ ▐▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓ ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓  ▓▓▓▓▓▓▓▓▓▓
+
+                           Trust math, not hardware.
+*/
+
+pragma solidity 0.5.17;
+
+import "./AbstractBonding.sol";
+
+
+/// @title ETH Bonding
+/// @notice Contract holding deposits from keeps' operators.
+contract ETHBonding is AbstractBonding {
+    /// @notice Initializes Keep Bonding contract.
+    /// @param registryAddress Keep registry contract address.
+    /// @param ethStakingAddress ETH Staking contract address.
+    constructor(address registryAddress, address ethStakingAddress)
+        public
+        AbstractBonding(registryAddress, ethStakingAddress)
+    {}
+
+    /// @notice Withdraws amount from operator's value available for bonding.
+    /// This function can be called only by:
+    /// - operator,
+    /// - owner of the stake.
+    ///
+    /// @param amount Value to withdraw in wei.
+    /// @param operator Address of the operator.
+    function withdraw(uint256 amount, address operator) public {
+        require(
+            msg.sender == operator || msg.sender == staking.ownerOf(operator),
+            "Only operator or the owner is allowed to withdraw bond"
+        );
+
+        withdrawBond(amount, operator);
+    }
+}