Add support for Doltgres indexes

[Hydrocharged]

• Add support for Poltgres-compatible indexes doltgresql#561

[coffeegoddd]

@Hydrocharged DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
17851bb	ok	5937457

version	total_tests
17851bb	5937457

correctness_percentage
100.0

[max-hoffman]

I like the doltgresql side, I mentioned this before but I think reimplementing map iters isn't the best separation of concerns. @zachmu is better at interface tradeoffs, but I think hiding all of the details behind a different Range.Matches is better than duplicating iterators. I could be missing some bespoke postgres logic that requires more control, but it looks like binary search callbacks to me. If "findStart" / "findStop" can't be rewritten for our existing star cursor/end cursor iters, I think it'd be straightforward to add those primitives to the tree layer rather than implementing in the index package. Even if we branch logic without interface niceties, it's just such a smaller surface area to test/way greater code reusability if we keep the map iter implementations narrow. Duplicating fixes to all of the noms iters for so long was inefficient, a bit of time spent avoiding that state again would be my vote. I apologize for the handwavy nature of these comments, hopefully it makes sense from prior context, and happy to dive into more specifics if you want.

[max-hoffman]

This seems like it's in a place that would be amenable to refactors. It'd be nice to maybe document why there is non-logical overlap between mysql and PG ranges, in case we want to try to make the arithmetic faster at some point. The start/stop semantics don't seem bad, but it is different from how we do it in MySQL, and if I had to guess it seems like both the MySQL and PG sides could use either interface with the right compensating filters.

[Hydrocharged]

dolthub/doltgresql#561

[coffeegoddd]

@Hydrocharged DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
24b49d0	ok	5937457

version	total_tests
24b49d0	5937457

correctness_percentage
100.0