…terface and implementation

scope restricted to associateBy() operation

Thanks for your hard work. I've looked over the various changes, looks good to me.

I just have two minor notes:

• As you already mentioned in Separate operation API from implementation #11 I would also prefer to just support user-defined operations via mixins. Dropping one function would make the API a bit simpler. I would prefer the name extendSequence(mixin) instead of augmentSequenceWithMixin().
• Please squash your commits into a single commit.

Thanks for the hint, wasn't aware of this option. I'll merge your changes soon.