Returns a pair [edit-distance edit-script] as result of comparision
of two maps am and bm.

The edit-distance is the number of assocs+dissocs necessary to
reproduce bm from am.

The edit-script is a map with three entries: 
  :+ sequence of k-v pairs that are only present in bm
  :- sequence of keys removed in bm
  :r sequence of k-v pairs that have changed in bm.

The edit-script can be used with patch to create bm from am.

Takes a map am and the result as produced by diff and returns a map
with all deletions, replacements and additions applied to the input
map.