interp: Switch to value type

Switch to value type in preparation for supporting primitives.

Signed-off-by: Pekka Enberg <penberg@kernel.org>
interp: iload_n

Signed-off-by: Pekka Enberg <penberg@kernel.org>
interp: iconst_n

Signed-off-by: Pekka Enberg <penberg@kernel.org>