Fixes the free variable problem on closure conversion

[rodrigogribeiro]

• Fixes the free variable problem on closure conversion
• Adds test cases.

[mbenke]

Free variable calculation for blocks should be corrected.

Crashing example (tries to add z to the closure in makeClosure; see also comments in code)

function addW (l: word, r: word) -> word {
     let rw : word;
     assembly {
	 rw := add(l,r);
     }
     return rw;
}


contract Bug {
    function main() -> word {
        return makeClosure(42);
    }

    function makeClosure(e : word) -> word {
        let f = lam (x : word) {
	    let y: word;
	    if (false) {
	      let z = 7;
	      y = z;
	    } else {
	      y = e;
	    }
	    return addW(e,x);  // this works
        };
        return f(1);
    }
}

[mbenke]

This may be slightly misleading, for a statement list like [let y = 42; return y], the free will include y even though it's bound locally. It is compensated for in closureConversion but I think it merits at least a comment in the class definition.

This approach also may result in incorrect closure - see comment about If statement

[mbenke]

For let y:word;if b then { let z = 7; y := z; } else ... we have z in free and not in bound. Then closure conversion tries to close over z.

[mbenke]

Looks good now, thanks!