Add applyMergeBy

[pgujjula]

Add a function

applyMergeBy :: (c -> c -> Ordering) -> (a -> b -> c) -> [a] -> [b] -> [c]

This is a little tricky since the underlying priority queue from pqueue does not support custom comparison functions.

[pgujjula]

https://www.tweag.io/blog/2017-12-21-reflection-tutorial/