Fix rust-mode lazy loading

This PR fixes the following error that happends when opening a .rs
file:

	File mode specification error: (void-function rust-mode)

It adds a stub function called `rust-mode-choose` that selects the
appropriate `rust-mode` version (prog or treesitter) based on the
environment.

Fixes rust-lang#528

rust-mode.el

@@ -71,12 +71,16 @@ instead of `prog-mode'. This option requires emacs29+."
     map)
   "Keymap for Rust major mode.")
 
-(if (and (version<= "29.1" emacs-version) rust-mode-treesitter-derive)
-    (require 'rust-mode-treesitter)
-  (require 'rust-prog-mode))
+;;;###autoload
+(defun rust-mode-choose ()
+  "Choose the appropriate rust-mode."
+  (if (and (version<= "29.1" emacs-version) rust-mode-treesitter-derive)
+      (require 'rust-mode-treesitter)
+    (require 'rust-prog-mode))
+  (rust-mode))
 
 ;;;###autoload
-(add-to-list 'auto-mode-alist '("\\.rs\\'" . rust-mode))
+(add-to-list 'auto-mode-alist '("\\.rs\\'" . rust-mode-choose))
 
 (provide 'rust-mode)
 (require 'rust-utils)