Prefer -when-let from dash

It's throwing errors in latest Emacs master.  See GH-52.

Author: fmdkdd

flycheck-rust.el

@@ -91,15 +91,15 @@ http://www.flycheck.org/en/latest/user/troubleshooting.html for \
 more information on setting your PATH with Emacs."))
     ;; metadata contains a list of packages, and each package has a list of
     ;; targets.  We concatenate all targets, regardless of the package.
-    (when-let ((packages (let-alist
+    (-when-let (packages (let-alist
                              (with-temp-buffer
                                (call-process cargo nil t nil
                                              "metadata" "--no-deps"
                                              "--manifest-path" manifest)
                                (goto-char (point-min))
                                (let ((json-array-type 'list))
                                  (json-read)))
-                           .packages)))
+                           .packages))
       (seq-mapcat (lambda (pkg)
                     (let-alist pkg .targets))
                   packages))))