Use small_integers in buildI instruction

From !17 (comment 2297752213):

The predefined values (static_booleans, static_characters, small_integers) are an optimization: they allow the garbage collector to remove, say, True nodes on the heap, by redirecting references to a True node in static memory.

Instructions like buildI should also be adapted to use small_integers. (buildB_b can also be adapted, but buildI_b shouldn't be: it is inefficient to check whether the value falls in the [0,32] interval. These values will be removed during GC.) A follow-up can be created for this.