Let cconv use :fun-body in special forms that need it.

* lisp/emacs-lisp/cconv.el (cconv-closure-convert): Drop `toplevel' arg.
(cconv-closure-convert-toplevel): Remove.
(cconv-lookup-let): New fun.
(cconv-closure-convert-rec): Don't bother with defs-are-legal.
Use :fun-body to handle special forms that require closing their forms.
* lisp/emacs-lisp/bytecomp.el (byte-compile-file-form, byte-compile):
Use cconv-closure-convert instead of cconv-closure-convert-toplevel.
(byte-compile-lambda, byte-compile-make-closure):
* lisp/emacs-lisp/byte-lexbind.el (byte-compile-maybe-push-heap-environment):
Make sure cconv did its job.
* lisp/emacs-lisp/byte-opt.el (byte-optimize-lapcode): Check stack-depth
before using it.
* lisp/dired.el (dired-desktop-buffer-misc-data): Don't use a dynamic var as
function argument.

lisp/emacs-lisp/byte-lexbind.el

@@ -585,6 +585,7 @@ proper scope)."
 	    (= nclosures byte-compile-current-num-closures))
 	;; No need to push a heap environment.
 	nil
+      (error "Should have been handled by cconv")
       ;; Have to push one.  A heap environment is really just a vector, so
       ;; we emit bytecodes to create a vector.  However, the size is not
       ;; fixed yet (the vector can grow if subforms use it to store