Rename refactoring, or something like that

[Dmitry Gutov]

We have "something like that" now, implemented as an xref command acting 
on a list of search results. To try it out:

1. M-x xref-find-references, or M-x project-find-regexp. Input something 
real, get a list of matches.

2. In the results buffer, press `r'.

3. Somewhat counter-intuitively, press RET to agree to `.*', then enter 
the replacement string, and press RET again.

4. Press `y' or `n' a few times, to choose which of the matches to 
actually replace. M-x save-some-buffers, then `!', to save the changes.

5. Feel hugely motivated, and write a better UI for all of that.