Pin operator not taken into consideration in metadata builder

[lukaszsamson]

The pin operator (^) is not taken into consideration so variables can be wrongly marked as definitions. E.g.

def my_fun(a, b) do
  case a do
    ^b -> b
  end
end

variable b will be treated as redefined in the case clause, but in fact, it is the same unchanged variable as the one defined in the function header. I suggest fixing it in a follow-up PR.

Extracted from #177