Added Adapter#get that takes a feature.

The goal is to make it easier to preload features all at once using
adapter logic, rather than all kinds of caching middleware and such.

Eventually, gates will not have toggles and not know how to save
themselves. Everything will go through adapter and the feature will
interact with it directly.
Rename feature values variable from adapter get.
Move feature into specs that need it rather than global feature for spec.
Move feature awareness to gate.
Move feature adding to feature from gate.
Just inspect gate value.
Use value in gates over toggle.value.

Currently value just delegates to toggle value anyway.
Remove toggle from actor.
Minor internal change to group gate.
Remove toggle from group.
Remove toggle from % of random
Remove gate from % of actors.
Remove toggle from boolean gate.
Reduce the supported "truthy" values.
Remove toggle from gate base class.
Completely remove toggle and everything about it.
Run all specs for all changes.
WIP: moving towards get, enable, disable for required adapter methods.
Remove enable and disable from gates.
Remove all traces of Key class.

No longer needed.