Demixinify

[chriskrycho]

• Inline filter-params mixin into its only user
• Replace parent-name mixin with simple getter and util

[chriskrycho]

Not sure why Percy failed here, either. 🤔 Trying the "merge in master and see if that helps" dance.

[chriskrycho]

All right, since the last commit here was the one which changed the FastBoot-related behavior, I've dropped that one and will try to take a further swing at it on a different PR; I also extracted what was accidentally the first commit here and extracted it to #830.

[chriskrycho]

There we go. I'll tackle the transitionTo and the last mixin separately!