Many mechanisms to track possible iname nestings

There are at least three mechanisms in loopy currently to track potential nestings of inames:

• get_visual_iname_order_embedding (used for printing)
• find_loop_nest_{with,around}_map (used in the scheduler)
• has_schedulable_iname_nesting (used to fix up indexed CSEs)

Potentially, that's two too many. We should figure out what they're each good at and if some are expendable.

cc @kaushikcfd @mattwala @dokempf