Make1_generic
is for blitting between two values of the same container type that's
not fully polymorphic (in the sense of Container.Generic).
val blit : ('a t, 'a t) Blit_intf.blit
val blito : ('a t, 'a t) Blit_intf.blito
val unsafe_blit : ('a t, 'a t) Blit_intf.blit
val sub : ('a t, 'a t) Blit_intf.sub
val subo : ('a t, 'a t) Blit_intf.subo