Improve tuple handling

[montyly]

Convert structure types to list of types if the lvalue is a tuple.

This specific case is needed in case an external call to a state variables that is a structure.

I haven't found other situations where tuples could appear from external calls, but I might have missed some cases.

Fix the second issue raised in #529.

Tested on

library Lib{

    function t() public returns(uint, bool, uint){
 
    }
    
}


contract Called{
    function f() public returns(uint, bool, uint);


    struct St{
        uint a;
        bool b;
        uint c;
    }

    St[] public sts;

}

contract Tuple {

  function test_low_level()
    public
  {
    bool success;
    address dest;
    bytes memory returnValue;
    (success, returnValue) = dest.call("");
    (, returnValue) = dest.call("");
  }

  function get() internal returns(uint, bool, uint);

  function test_internal() public{
        uint a;
        bool b;
        uint c;
        (a,b,c) = get();
        (,b,c) = get();
        (,b,) = get();
  }


  function test_external_call(Called dst) public{
        uint a;
        bool b;
        uint c;
        (a,b,c) = dst.f();
        (,b,c) = dst.f();
        (,b,) = dst.f();
  }

  function test_external_st_array(Called dst) public{
        uint a;
        bool b;
        uint c;
        (a,b,c) = dst.sts(0);
        (,b,c) = dst.sts(0);
        (,b,) = dst.sts(0);
  }


}