Add cider-inspector-def-current-val to cider-inspector

[jpmonettas]

Add cider-inspector-def-current-val command to cider inspector so you can define a var with the inspector current value.
Also binds d in the inspector mode map to this command and adds "Define var" to the inspector menu.

Depends on clojure-emacs/cider-nrepl#653

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[bbatsov]

Nicely done! Thanks! 🙇