Directly creating the node isn't necessary anymore, the token only needs
to be parsed. This makes it consistent with the other scalar parsing
methods and removes the need to pass $arguments around.
The parser didn't account for the additional newline after the content of doc strings, which is left there by the tokenizer for some reason. Additoinally esacape sequences were parsed in nowdoc strings.
Additionally this contains some minor changes to the grammar: Some _list nonterminals were refactored to have the possible single elements in a reparate rule and only assemble those single elements. (This reduces duplication and gives better assignment of line number context.)
