[BUGFIX] Create _super when applying mixins. Use __nextSuper in wrappers

This sets _super on the context of all Ember objects, and also sets _super
on mergedProperty contexts if they have a property that is a function.

Fixes #3523, discussed in #3683
[BUGFIX release] Reduce read-only calls to meta
[BUGFIX release] Pass meta into watchKey when possible
[BUGFIX release] Reduce even more meta calls by passing it along in other places
[BUGFIX release] Reduce even more meta calls by passing it along in other places (doh, missed one)
[BUGFIX beta] fixed issue with transition.resolvedModels

Already-resolved parent models weren't showing
up in modelFor hook post-refactor

cc @wagenet
Merge pull request #3683 from mixonic/super

[BUGFIX] Ever-present _super, avoids infinite loop case