perf(lib): optimize doom-path

Consecutive expand-file-name and recursive apply's can be expensive, so
the function has been simplified to rely more on file-name-concat. This
does change one trait about it, however: absolute paths in SEGMENTS no
long reroot the whole path, and are concatenated as ordinary file
segments.

The performance benefit is more pronounced on Emacs 28+, and will be
even more so when Doom later starts byte-compiling its libraries.

lisp/lib/files.el

@@ -38,19 +38,19 @@ This is used by `file-exists-p!' and `project-file-exists-p!'."
 
 ;;;###autoload
 (defun doom-path (&rest segments)
-  "Constructs a file path from SEGMENTS.
+  "Return an path expanded after concatenating SEGMENTS with path separators.
-Ignores `nil' elements in SEGMENTS."
+
-  (let ((segments (remq nil segments))
+Ignores `nil' elements in SEGMENTS, and is intended as a fast compromise between
-        file-name-handler-alist
+`expand-file-name' (slow, but accurate), `file-name-concat' (fast, but
-        dir)
+inaccurate)."
-    (while segments
+  ;; PERF: An empty `file-name-handler-alist' = faster `expand-file-name'.
-      (setq segment (pop segments)
+  (let (file-name-handler-alist)
-            dir (expand-file-name
+    (expand-file-name
-                 (if (listp segment)
+     ;; PERF: avoid the overhead of `apply' in the trivial case. This function
-                     (apply #'doom-path dir segment)
+     ;;   is used a lot, so every bit counts.
-                   segment)
+     (if (cdr segments)
-                 dir)))
+         (apply #'file-name-concat segments)
-    dir))
+       (car segments)))))
 
 ;;;###autoload
 (defun doom-glob (&rest segments)