Add support to top level variables

[montyly]

Fix #995

Example:

# top_level_variable.sol
uint constant A = 10;

struct St{
    uint[A] b;
}

function f() returns(uint){
    return A;
}

contract T{
    function g() public{
        f();
    }

}

# top_level_variable2.sol
import "./top_level_variable.sol";

contract T2{

    function h(uint[A] memory) public{
    }
}

TODO:

• Add the parsing tests
• Do more tests on the parsing on real contracts