Store all current ivy candidates into the kill ring

I'd like to highlight a new command added to ivy-mode today: ivy-kill-ring-save. It allows to store all current candidates to the kill ring. This could have a number of uses.

Scenario 1

Suppose you want to learn some Elisp, specifically all functions that start with forward-. Then call counsel-describe-function with "forward-" as input and press M-w. Then just yank this into some buffer, org-mode for instance, and go through the functions one-by-one takings notes on the way:

forward-comment
forward-paragraph
forward-thing
forward-symbol
forward-visible-line
forward-word
forward-same-syntax
forward-whitespace
forward-button
forward-line
forward-list
forward-sentence
forward-point
forward-to-indentation
forward-ifdef
forward-char
forward-sexp
forward-page

Scenario 2

Suppose you want to store a subset of projectile-find-file names that match a pattern. Just press M-w and those file names will be on the top of the kill ring. Of course, you need to have this setting on:

(setq projectile-completion-system 'ivy)

This scenario should apply to basically any place where you're completing file names, like counsel-locate or counsel-find-file. Also find-file-in-project, which uses ivy for completion if it's installed.

Outro

Note also that the command comes for free, since normally nothing is done by M-w when the region isn't active. When the region is active, this command will store the selected text instead of matched candidates.

Thanks to @drorbemet for giving me the idea in #197.