Discuss how to manage common utility functions

This discussion is broken out of https://github.com/enthought/traits/issues/1239. That issue started out as a specific proposal to add the non-Traits-related utility functions `xgetattr` and `xsetattr` to `traits.api`, and turned into a more general discussion. We should continue that more general discussion here.

Summary: at issue is where to put functionality representing common non-package-specific tasks found across ETS and ETS-using projects. Apart from `xgetattr` and `xsetattr`, examples include time and timezone handling, slugification of strings, and common testing utilities.

See enthought/traits#1239 for previous discussion on the topic.