Experimental: more knob template shenanigans

[hysw]

DO NOT SUBMIT This is a thought experiment or template shenanigan as I'd like to call it...

Slightly more flexible knob definition

Note:

• KnobFlag<T> should also be directly replaced by KnobValue<Spec<T>>
• KnobDropdown probably need separate implementation, i.e. KnobIndexedValue<Spec<T>>