Anaphoric macros: increase visibility

[Tino Calancha]

Hi,

I think anaphoric macros are a nice idiom; occasionally
they allow to write very concise and clear expressions.

AFAICS, in the Emacs tree they are just introduced
in ibuf-macs.el (`ibuffer-aif', `ibuffer-awhen').

To use these macros in Emacs code we must:
1) Define them again (code duplication).
2) Or require ibuf-macs, which it sounds not conventional.

It might have sense to move these general purpose macros
into a more visible location (subr.el or subr-x.el).

We might add other useful macros as well, e.g., awhile.

What do you think?

Tino