Add reload in place capability

[igor-makarov]

As a user of TableViewDiffCalculator, I'd like to minimize unnecessary animations.

This PR transforms subsequent delete+insert calls to reload calls.

This also addresses #60.

[jflinter]

Thanks for the PR! While I'd very much like this functionality to exist, I have some issues with this implementation:

• Information shouldn't be passed around in strings a la encodedValue; this loses a lot of type safety and makes the code harder to understand.
• These changes should also appear in CollectionViewDiffCalculator.
• This implementation will only handle insertions/deletions that are adjacent to one another, but Dwifft orders the edits by processing all deletions followed by all insertions. Thus many legitimate reloads would be missed by this approach. What is truly needed is the ability to "coalesce" an array of DiffSteps containing arbitrarily-ordered insertions and deletions into one containing insertions, deletions, and (presumably) some sort of new reload case. Unfortunately this is challenging with the involved index math.

The last of those points is the most serious, so I'd suggest thinking about it pretty carefully (and posting your thoughts here) before starting to implement anything. (It's a hard problem, which is why I've avoided it for so long.)