Add stream to Parser.Error, enabling nicer error messages.

[AmosNico]

Add the stream to Parser.Error as follows:

protected class Parser.Error (ε σ : Type _) (τ : outParam (Type _)) [Parser.Stream σ τ] where
  unexpected : σ → Stream.Position σ → Option τ → ε
  addMessage : ε → σ → Stream.Position σ → String → ε

This makes it possible to use (parts of) the stream when displaying error messages. This is particular useful in string parsers, where it would enable displaying the line where the error occurred. While I have not tested performance yet, I expect that the change should not effect performance to much, assuming that the underlying stream is not modified.
This is especially the case for #124, but even without this pr I think at most some references to the underlying String/Array/... and some positions will be duplicated.

If there is interest, I would be happy to make a pr adding nicer error formatting for string parsers to the library.