Provide a way to merge records and fields.

We should provide a way to merge two or more records, fragments of records, or mergeable lists or items in mergeable lists, so that we get the data from both mergeables, and return a merge of those where all keys that had values in in one but not the other, or had the same value in both, are filled in with the previous value, and (optionally) either raise a ConflictsException with all keys that had unequal values, or point to one of the mergees and let its values win in any conflicts.

This will allow building UI to merge conflicting records pretty easily, and allow us to actually merge two mergeable lists, which is their intended but sadly unfulfilled purpose, so far.