Fix Code.Typespec.spec_to_quoted/2 incomplete variable type collection

[AlexGx]

Code.Typespec.spec_to_quoted/2 skips type vars nested in annotated spec arguments

Erlang stdlib array

iex> h :array.default/1
...
  @spec default(array :: array(type)) :: value :: type

Expected

@spec default(array :: array(type)) :: value :: type when type: var

Elixir module

defmodule Repro do
  @spec foo(arg1 :: [t], arg2 :: [t]) :: atom() when t: var
  def foo(_, _), do: :ok
end

iex>  h Repro.foo/2
...    
    @spec foo(arg1 :: [t], arg2 :: [t]) :: atom()

Expected

@spec foo(arg1 :: [t], arg2 :: [t]) :: atom() when t: var`

when TYPE_VAR: var part is missing because :ann_type node is ignored

[josevalim]

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