Improve clojure-ts--node-child-skip-metadata performance by ~6x

[kommen]

I noticed a severe lag when invoking imenu in larger clojure namespaces with clojure-ts-mode.
Traced it down to an inefficeny in clojure-ts--node-child-skip-metadata. With this fix invoking imenu gets very snappy. As this function is also used by indendation and font-lock, it should also improve the performance there as well.

While at it, I added correctness tests for it. I found the docstring a bit confusing, I hope I didn't missunderstand it?

Using this to benchmark shows a 6.5x improvement on my machine:

(let ((old-impl (lambda (node n)
                    (let ((value-nodes
                           (seq-filter
                            (lambda (child)
                              (string= (treesit-node-field-name child) "value"))
                            (treesit-node-children node t))))
                      (seq-elt value-nodes n))))
        (iterations 500))
    (with-clojure-ts-buffer "(defn ^:private foo [x] x)"
      (let* ((node (clojure-ts-test--first-list-node))
             (bench (lambda (fn)
                      (garbage-collect)
                      (let ((start (float-time)))
                        (dotimes (_ iterations)
                          (funcall fn node 0)
                          (funcall fn node 1))
                        (- (float-time) start))))
             (median (lambda (times)
                       (let ((sorted (sort (copy-sequence times) #'<)))
                         (nth (/ (length sorted) 2) sorted)))))
        ;; Warm up
        (funcall bench old-impl)
        (funcall bench #'clojure-ts--node-child-skip-metadata)
        ;; Collect samples
        (let* ((old-times (mapcar (lambda (_) (funcall bench old-impl))
                                  (number-sequence 1 7)))
               (new-times (mapcar (lambda (_) (funcall bench #'clojure-ts--node-child-skip-metadata))
                                  (number-sequence 1 7)))
               (old-median (funcall median old-times))
               (new-median (funcall median new-times)))
          (message (concat "node-child-skip-metadata benchmark (%d iterations):\n"
                           "  old: median=%.3fs samples=%s\n"
                           "  new: median=%.3fs samples=%s\n"
                           "  speedup: %.1fx")
                   iterations
                   old-median (mapcar (lambda (t) (format "%.3f" t)) old-times)
                   new-median (mapcar (lambda (t) (format "%.3f" t)) new-times)
                   (/ old-median new-median))))))

---

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• The commits are consistent with our contribution guidelines.
• You've added tests (if possible) to cover your change(s). Bugfix, indentation, and font-lock tests are extremely important!
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• You've updated the changelog (if adding/changing user-visible functionality).
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Thanks!

[bbatsov]

Thanks!