Rationalize/reuse parsers?

[jamescheney]

#517 is a good idea but seems like an occasion to rethink/rationalize the various lexers/parsers we currently support (Links, json, XML).

The XML lexer/parser was added a while ago and is not used internally in Links but is exposed as a library function parseXml. I don't know if we have any active use cases where we need this.

@SimonJF has a use case for an HTML parser (i.e. to handle auto-closed tags and non-quoted attributes), which is currently implemented in plain Links using parser combinators, and is probably a lot slower than an ocamllex/yacc parser would be. We could in principle do the same thing to parse plain XML.

The JSON lexer/parser exists to allow parsing JSON messages but is not accessible from Links. Might it be useful to support a parseJson library function - especially if we also were to support sending RPC-calls to other REST-style services (or serving such calls from Links)?

Alternatively, if we want to support efficient parsing of all three either internally or exposed as Links library functions, I can also imagine that there might be standard JSON, XML or HTML parsing libraries available for OCaml that we could simply reuse here. From a quick google of OPAM, yojson appears to support JSON parsing and markup appears to support HTML and XML parsing, and we already depend on both libraries.