Move p:www-form-url* steps to functions?

[xatapult]

The p:www-form-url* steps are weird ones. They do nothing with documents flowing through. They're actually just functions in disguise.

Why don't we implement them as XProc extension functions, that would make it way easier to use their functionality.

Whether we remove the existing steps also is a matter of backwards compatibility, but my guess is they're not often used.

[ndw]

That's definitely a possibility. I think there was a time, early on, when it wasn't clear how practical it would be for implementors to provide new functions. But we've certainly embraced that now.