Preview implementation of "coalesce" function

Here's a first draft of a "coalesce" function as I mentioned in discussion about str2bool in issue 14908:

  https://projects.puppetlabs.com/issues/14908#note-4

I am submitting this more as a request for review than an actual pull request; there are a few other cases that it should test & handle, but I wanted to see if the idea was acceptable before investing more time in it. If you like it I will open an RFE and submit it properly.