Proposal: more general unionWith for Data.Map

[John Meacham]

Oh yeah, I didn't mean we should actually replace the implementations,
I was just demonstrating that merge is truely a more general operation
and provide evidence it was useful.

The most recent use case for me dealt with a lattice (in the
mathematical sense) class, where the result of a meet or join could be
a different type than the arguments... or maybe it was a boolean
algebra class... hrmm.. or was it an implemenation of approximations
of probability distributions where the map contained representative
samples... sigh.. I juggle too many projects at once.. :)

   John