`ORDER BY` parser

Would a specific `ORDER BY` parser fit into this crate? It would generate an iterator which yields `Result`. I have developed a rudimentary version of this, which I am using in production. I could further polish it and open a PR if this would be of interest. I am just repeatedly calling `parse_order_by_expr` and looking for `EOF`.

Start by reading the parser entry point for parse_order_by_expr and the existing ORDER BY grammar, then compare how ParserError and EOF are handled by related parser APIs. The scope still needs agreement on the iterator interface; done would include an accepted API and tests for multiple expressions, EOF, and parse errors.

Written by the indexing model from the issue text.