Create a `pure function` checker, that will check if functions are pure.

[trezm]

Create a TDPureFunctionChecker that will throw errors if:

• Properties on external variables are changed, e.g.

var s = {}

function() /* t:number */ {
  s.a = 1;
  return 1;
}

• Parameters are changed, e.g.

function(/* t:Object */ a) /* t:number */{
  a.key = 1;
  return 1;
}

• If a function does not have an explicit return, e.g.

function() /* t:undefined */ {
  console.log('hi');
}