Add support for undef-all op

[yuhan0]

See clojure-emacs/cider-nrepl#698.

Added prefix argument behaviors to cider-load-buffer, -load-file, -load-all-files, and -eval-ns-form commands, since these are likely to throw errors when some redefined namespace alias conflicts with the hidden REPL state. Conceptually something like a "hard reload".

Also adds a standalone command cider-undef-all, which is somewhat less useful on its own and would usually be followed by a load anyway. Proposed keybinding: C-c C-M-u by extension of C-c C-u.

Also fixes a bug in my last PR #3185 (I should really avoid doing these last minute refactorings 😶)

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Thanks!

If you're just starting out to hack on CIDER you might find this section of its
manual extremely useful.

[bbatsov]

I don't see this mentioned in the docs. Perhaps it wasn't there in the first place?

[yuhan0]

Added, thanks for the catch!