Piece-wise linear interpolation from float-like types to float.
This type constructor is how we expose, for instance, that Make(K)(V).t and
Stable.V1.Make(K)(V).t are the same type (as long as Stable.V1 is current).
Likewise, if K0.t = K1.t, then Make(K0)(V).t = Make(K1)(V).t.
Key and Value.