layer merge support

[propensive]

There are a variety of occasions when users will need to merge layers. We can already diff two layers, getting a list of differences between two layers, but we would like to selectively choose which differences to merge and which not to.

This is a large piece of work, and will facilitate automatic merging of differences originating in different layers, and also the storage of named variations between layers. It primarily requires changes to the Diff mechanism to allow serialization (and deserialization) of each difference.