added some helpers to make it easier to open or create a pail

These are 3 functions I use a lot. Find-or-create is especially nice in practice. I noticed that  I could just use create with fail-on-exists set to false, but it seemed clearer to try opening the pail first.

It could be that find-or-create should take the optional arguments for create as well but it didn't seem as important.

I wasn't sure how to begin to test them.