Piece-wise linear interpolation from float-like types to float.
This type constructor is how we expose, for instance, that
Stable.V1.Make(K)(V).t are the same type (as long as
Stable.V1 is current).
K0.t = K1.t, then
Make(K0)(V).t = Make(K1)(V).t.
Note that applications of the following functors are only as stable