Type Inference for If statements

[MichaReiser]

Add support for IF-Statements to the type Inference algorithm. The following edge cases need to be
supported, as they are widely used.

Type inference needs to be branch specific

function (x) {
  if (typeof(x) === 'string') { // inside the branch x is of type string
    x = parseInt(x, 10); // type changes from string to int
  }
  return x * x; // the type of x is always number
}

The possible types of a variable after an if statement need to be the most common between all branches

if (y > 10) {
  x = 10;  // x is typeof number
else {
  x = null; // x is typeof Null
}

return x + 10; // x is typeof Maybe<number>