Add `clojure-find-def` for var definitions.

This is like `clojure-find-ns`, but returns the `def`-ing macro and var names for the enclosing form at point.
This doesn't currently capture all valid symbol names because `clojure-mode` doesn't seem to set the clojure syntax table for the `\w` class. Without this, we'd have to replace `\sw` to `[A-Za-z0-9*+!-_?]` here.