Mark code that uses improper lists as generated

[al2o3cr]

Addresses #549 by marking code that creates improper lists as generated: true.

If it's preferable to explicitly opt-out of the specific Dialyzer warnings (using @dialyzer), that's an easy change.

[al2o3cr]

This is a common pattern: the code explicitly makes a definitely improper list; this is fine for iodata and saves a cons cell.

[al2o3cr]

This one covers clauses like:

      defp decode_rows(<<?D, rest::binary>>, _, _, rows) do
        {:more, [?D | rest], rows, 0}
      end

in the code; is there a performance difference between creating an improper list which is immediately turned back into a binary and passing along the binary directly (maybe related to "small piece of a big buffer" issues?)

      defp decode_rows(<<?D, rest::binary>> = whole_thing, _, _, rows) do
        {:more, whole_thing, rows, 0}
      end

[al2o3cr]

This covers the code starting at line 500:

        case Postgrex.Types.fetch(oid, types) do
          {:ok, {:binary, type}} ->
            unquote(oids) = rem - 1

            case [type | types] do
              unquote(dispatch)
            end

That case [type | types] do code is tricky - it looks like a boring "prepend to list" operation, but AFAIK types is never a list (it's a Postgrex.Types.state!) so it's always making an improper list.

[josevalim]

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