Unkeyed list diffing enhancements

## Feature Request

We should implement more look-ahead optimizations for unkeyed lists. Sometimes you omit a key or are lazy and we can likely make the vast majority of cases as good as having a key. Things like look-ahead, look-back, index/hash caching, etc.

Also, auto-hashing of an unkeyed template's dynamic portions in small cases could allow us to generate keys implicitly from data, perhaps dropping the need for keys completely.

This would let us do things like the counters example keylessly but with keyed performance which would be a huuuge win for basic/lazy usecases.