Consider integrating an IDL parser

[domenic]

Moved from https://www.w3.org/Bugs/Public/show_bug.cgi?id=26343. This would potentially allow:

• Automatic cross-referencing without having to insert <span>s everywhere
• Better handling of partial interfaces and backlinks, which is currently done via hacky perl scripts in html-build (cf. https://www.w3.org/Bugs/Public/show_bug.cgi?id=26319)

I would imagine we would do this similar to how we did syntax highlighting, by shelling out to an external program.

I don't know whether this issue is very high priority. But it could be nice.