Expose the main Parsec combinators from the library

[ezrakilty]

Following https://github.com/haskell/network-uri/issues/56, this is a proposal to expose a couple of main Parsec parsers so that users can compose further combinators on top of them.

My inclination is to be conservative and start small in terms of which parsers we expose.

Note also I exposed the existing parsers under new names, so that (a) they match the corresponding parseX functions, and (b) they have ...Parser in their names so they're easily identified as Parsec items.