Make some functions constant

[shamatar]

It should be possible to make some functions constant, at least simple ones like an empty state. It will require to add extra constructors because runtime feature detection is obviously not constant. With further evolution of constant functions in rust it should be possible to precompute State instances if one always uses some keying.

[oconnor663]

I wonder if this would make more sense as a separate crate. Maybe there'd be no need for the State model anyway, and everything could be all-at-once functions?