Decreasing the amount of functions duplicated.

index_list was redefining nth_error from the standard library.
upd_nat (in Machine.v) was redefining update_list (from Utils.v)
upd (in Machine.v) was redefining update_list_Z (from Utils.v)
etc...

Changed the order of arguments in a way that I hope to be natural to many
programmers, and is at least to me :)