Use "annex" in Wsgi class

When referring to the right-side of a node in the Wsgi class, use the
same terminology as the Locator class (i.e. an annex, not a "pathway").
Use assert for final Wsgi rule type check

This is done so that in the future the assert() call can be optimized
out during deployment (as a non-dict/int/string here would be a logic
error).
Consistent use of framework constants

In general, all constants should be tied to one of the classes and
should be publicly visible (and as a special case, MSG_NOTFOUND will be
made available as a module level alias to Wsgi.MSG_NOTFOUND, since it
will be so commonly needed by outside modules).