kern lookup order can vary in unimportant ways in some cases

This is a cause of a diff for [liter.glyphs](https://github.com/skugiz/liter): the two compilers are generating the same lookups, but they end up being in a slightly different order in the lookup list, which causes some indices to be different in the GPOS.

This is already mostly handled by normalization, but noramlization does currently leave the lookup order in place. The most obvious solution here is to remove the set of lookups completely in the ttx diff at all for at least kern/dist.

Start by tracing normalization and the ttx diff handling for kern/dist, focusing on where lookup order is preserved. Compare the generated lookup lists for the liter.glyphs case and verify that equivalent lookup sets no longer produce a diff solely from ordering.

Written by the indexing model from the issue text.