Fix namer to support recursive variable declarations

[b-studios]

See @phischu's talk at Shonan.

The type of mutable variables cannot mention the variable itself. This should be allowed.

[b-studios]

@phischu could you add the example from your talk here?

[phischu]

The following program typechecks:

def main() = {
  var x = box { () => 3 };
  x = box { () => x() };
  x()
}

The type of x is inferred to be () => Int at {x}. However, when I try to annotate this type:

def main() = {
  var x: () => Int at {x} = box { () => 3 };
  x = box { () => x() };
  x()
}

The program is rejected with "Could not resolve capture x".