Allow Psalm to add missing parameter types

[muglug]

Given the following code:

function takesString(string $s) : void {}

function shouldTakeString($s) : void {
  takesString($s);
}

with --add-missing-param-types, Psalm would return

function takesString(string $s) : void {}

function shouldTakeString(string $s) : void {
  takesString($s);
}

In this way we could approximate Hindley-Milner type inference, while improving the underlying PHP code.

It would do nothing with this input:

function takesString(string $s) : void {}

function shouldTakeString($s) : void {
  if (is_string($s)) {
    takesString($s);
  }
}