[Solidity] Fix Visibility

[ChenfengWei0]

fix the algorithm, e.g.

contract Base()
{
    function test1() public {}
    function test2() private {}
}

will now convert to

void sol_main()
{
    while(nondet_bool())
    {
        if(nondet_bool())  test1();
        // if(nondet_bool())  test2();  not include this, due to that it's a private function
}

Also, we add an option --no-visibility which forces verification of all functions (e.g. test2).