(bug 46366) Use SnakFormatter for diffs and summaries

This uses the new SnakFormatterFactory facility to allow
ClaimDifferenceVisualizer and ClaimSummaryBuilder to
operate on snaks in a generic way, without any knowledge about
data types or data values.

Change-Id: Ide1e947d9073f4648c4b97fd979c68c7ff48a984
Merge "(bug 46366) Use SnakFormatter for diffs and summaries"