Implement new `_comp_xfunc` and naming

[scop]

Per #539 (reply in thread)

New _comp_xfunc implementation, renaming of target functions, docs in doc/api-and-naming.md

[scop]

Docs done in ed168f0

[akinomyoga]

Thank you for working on these related changes. If there is anything I can do, please let me know as these big changes are something that I have originally triggered. For example, the implementation of _comp_xfunc and renaming of the public interfaces defined in bash_completion (while keeping the old names), etc. (edit: I just feel sorry that I'm leaving you everything, so if you would like to do it by yourself voluntarily that's just fine.)

[scop]

No problem, as far as I'm concerned it's first come first serve, whoever wants to implement something and starts work on it, gets it :) For example this one, feel free to grab it. And there are a few PR's pending for review.

Re the backwards compatibility names, I was thinking it could be great if those could be shipped e.g. in a separate file in /etc/bash_completion.d, but that doesn't work that well for files in completions/*. But perhaps we could do that for things where it does work.

[akinomyoga]

Thanks. OK, I'll work on _comp_xfunc.