Slot sequencing modes for filter-injectors

The filter-injectors used to always prefer to take items from the
beginning of the source inventory.  Because items also tend to get placed
at the beginning of an inventory, this could lead to a chest+injector
buffer only passing through a trickle of one type of item, where other
item types are flowing in and getting later inventory slots.  So now,
the sequencing of source inventory slots is configurable, per-injector,
by form.  In addition to the old priority mode, there's now a random
mode and a rotation mode.

Incidentally refactor the filter-injector code, to avoid duplication
between the itemwise and stackwise variants.