Move functionality from MixinBuilder to mixin decorators

[Justin Fagnani]

Currently MixinBuilder handles the caching of mixin applications and implementation of @@hasInstance which means that mixin consumers only get those features when they use mixwith.js, and not when they use the mixin function directly.

I'd like to move most of the functionality into mixin decorators, so that the author of the mixin adds application caching, @@hasInstance, delegation, de-duping, and possibly trait-like name-collision checks. Users of mixins then get these features no matter how they apply them.