Revert 'add context and flag argument to js_newstate' commit.
The context argument just adds clutter. The flag which was intended
for JS_DEBUG and/or JS_STRICT shouldn't be necessary.
js_newcfunction and js_newcconstructor need an extra argument, the
name of the function to use in stack traces.
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.
