Add a 'make unique key' function.

Essentially, force one of the keys (or an ordered list of keys) to be unique. When a duplicate entry is found, either:

a. the two dictionaries are 'merged'. where keys are in conflict, the 'first' or 'last' value is kept.
b. the last duplicate is kept. All others are discarded.
c. the first entry with the key is kept, all others discarded.
