Start with 2^128 instead of 2^256 eth

[samalws-tob]

Very simple code change. Start sender addresses with 2^128 eth instead of 2^256. This should prevent overflow over 2^256 while still preventing out-of-gas. Fixes the following test case which previously caused medusa to error:

contract SimpleBalanceDrain {
    function deposit() public payable {}

    function withdraw() public {
        uint256 amount = address(this).balance;
        payable(msg.sender).call{value: amount}("");
    }
}

Previously we got the following error:

error Encountered an error in the main fuzzing loop
‣ test chain state write error when adding tx to pending block: insufficient funds for gas * price + value: address 0x0000000000000000000000000000000000010000 have 4026804785036390882 want 18446744073709551615

This is because we overflowed a uint256 when adding onto 2^256-1