Indexed snapshot map

[maurolacy]

Closes #255.

Straightforward "clone" of IndexedMap.

[ethanfrey]

You need to expose the SnapshotMap methods.
A straight copy is okay, but maybe a bit hard to maintain.

---

One thought (which may be misguided, but please consider) is to make some IndexedCollection which takes a trait for primary. It needs:

trait Indexable {
  save(&self, ...);
  remove(&self, ...);
  may_load(&self, ...);
  // and maybe
  load(&self, ...);
}

Then you can wrap either a Map or IndexedMap as "Indexable".

In order to expose the custom methods on SnapshotMap, one could make pub primary: T <T: Indexable>, and call indexed.primary.load_at_height(...) for example.

(Note, maybe it is best that Indexable uses different names, so the common names don't require importing a trait, just a trait wrapper that defers to those, but you get the idea).

Then again, the majority of the work is done inside the Indexes (not IndexedMap), so maybe this is the simplest and most stable solution.

[ethanfrey]

Looks good, so this is the basic implementation, and I look at #272 for usage.

Do you want to merge those together and then go to master? Or merge this first, then the other when it is ready?

In either case, happy to review it as is, and give some pointers. But let's revert the pub primary once we have finalized the design and before merging to master.

[ethanfrey]

Once #272 passes CI with full tests, I would just address this comment as part of this PR: #272 (comment)

Otherwise, looking good

[maurolacy]

Thanks for the comments. I would merge this first, as it's done and contained.

And yes, we can always revert the pub primary in the other branch, when the proper methods are exposed.