Export URI parser

The library exports `String -> Maybe URI` functions, but this is not easy to compose with other parsers. For example, I'm writing a tokeniser that needs to identify URIs and a bunch of other stuff. I have to do some tedious gymnastics to fit `parseURI` in, but if I had access to the actual `Parser` itself, it would be much easier.

Start by locating the exported parseURI function and the parser it wraps. Check existing parser-related tests, then expose a composable Parser entry point alongside the String -> Maybe URI API; done means clients can combine it with other parsers without adapter gymnastics.

Written by the indexing model from the issue text.