Add XForms 1.1 id() function

Note that id(x) is already implemented in formsPlayer. What remains is to implement the id(x, y) functionality.

What does the id(x) function do? I imagine that x is a string representing an ID reference, but where does this function search for that ID: in the host document, in the first instance, in all instances, some combination of these, or none of the above?

In the in-scope instance.