Add userdata class.

js_newuserdata(J, tag, ptr) creates a userdata object, wrapping the ptr.
It takes the prototype for the new object from the top of the stack, just
like js_newcconstructor does.

js_touserdata(J, tag, idx) extracts the userdata pointer. Throws a
TypeError if the object is not a userdata object with a matching tag.

js_isuserdata(J, tag, idx) checks if a value on the stack is a userdata
object with a matching tag.