Introduce implicit keying

[chinedufn]

This commit introduces implicit keys to siblings that are being diffed.

This was introduced to solve for the following case:

start: <div> <span><input /></span> </div>
end: <div> <img /> <span><input /></span> </div>

Before this commit, if the input in the above example was focused at
the start, it would lose its focused after the patches were applied
since the span would have gotten replaced by the img.

Now all elements with the same tag are implicitly keyed such that
the diffing algorithm will send up generating patches that will move
elements instead of replace them, when possible.

Note that there is a cost to this, since the longest increasing
subsequence algorithm that we're using to diff keyed siblings is
`O(n log n).

This commit makes it such that n will always be the number of child
elements, now that all child elements have either an explicit or
implicit key and will all pass through our LCS pass.

Note that the n is the number of siblings across the chldren of two
different nodes, not the number of nodes in the entire dom tree.

So, for most realistic use cases n should be relatively small, since
most use cases don't have large lists of sibling nodes.