Improve performance of Relation::merge

[sapphire-arches]

Instead of doing the easy thing and just doing a sort-dedup, we make the following optimizations:

• if either element list is empty, just return that one
• if the first element of one list is > the last element of the other list, just extend the storage for the other list and append.
• perform a proper zipper merge of the two lists.

This should improve performance of merging by ~20% in some cases

[frankmcsherry]

Looks good, thank you!