`this.__inst' within trait methods will now correctly resolve to the public
visibility object of the class we're mixed into, rather than
`undefined'. This behavior is consistent with the rest of the system.
* lib/ClassBuilder.js (initInstance): Add `inst' to private metadata. This
is the public visibility object.
* lib/Trait.js (tctor): Initialize concrete trait `__inst' to aforementioned
`inst' metadata value.
* test/Trait/LinearizationTest.js: Add respective test.