Fix Handling of Higher-Order Functions

[dmzimmerman]

The current OCaml-Solidity parser does not properly handle the use of higher-order functions. For example, given this code:

contract A {
    uint256 internal a;

    function accessibleInternally(int256 b) public {
        b = b + 1;
    }

    function hof(function(int256) somfunc, int256 a) internal {
        a = a + 1;
    }

    function useHof() public {
        hof(accessibleInternally, 10);
        a = 1 + 1;
    }
}

it returns the following error: Type error: Invalid type for argument in "function call". Invalid implicit conversion from "function(int256) nonpayable " to "function(int256) nonpayable " requested. This is because it doesn't realize that a public function is an acceptable substitute for the internal function expected as a parameter to hof.

This PR fixes this issue (and other similar type conversion issues) with higher-order functions by implementing the allowed conversions for function types and mutabilities; it also includes several test cases to exercise the functionality.

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[dmzimmerman]

I'm not sure why it thinks I haven't signed the CLA; I signed it for the other PR (and just went through it again now).