(macroexp--unfold-lambda): Obey the lexbind semantics

While at it, rework the code so as not to rely on an
intermediate rewriting of (funcall (lambda ..) ...)
to ((lambda ..) ...) since that forms is deprecated.

* lisp/emacs-lisp/byte-opt.el (byte-optimize-funcall): Unfold lambdas
instead of turning them into the deprecated ((lambda ..) ..).
(byte-optimize-form-code-walker): Don't unfold ((lambda ..) ..) any more.
(byte-compile-inline-expand): Revert to non-optimized call if the unfolding
can't be optimized.

* lisp/emacs-lisp/bytecomp.el (byte-compile-form): Don't unfold
((lambda ..) ..) any more.

* lisp/emacs-lisp/cl-macs.el (cl--slet): Remove workaround.

* lisp/emacs-lisp/disass.el (disassemble): Make sure the code is
compiled with its own `lexical-binding` value.

* lisp/emacs-lisp/macroexp.el (macroexp--unfold-lambda): Make it work
both for ((lambda ..) ..) and for (funcall #'(lambda ..) ..).
Be careful not to move dynbound vars from `lambda` to `let`.
(macroexp--expand-all): Unfold (funcall #'(lambda ..) ..) instead of
turning it into ((lambda ..) ..).  Don't unfold ((lambda ..) ..) any more.

lisp/emacs-lisp/cl-macs.el

@@ -251,10 +251,8 @@ The name is made by appending a number to PREFIX, default \"T\"."
       (if (macroexp--dynamic-variable-p (car binding)) (setq dyn t)))
     (cond
      (dyn
-      ;; FIXME: We use `identity' to obfuscate the code enough to
-      ;; circumvent the known bug in `macroexp--unfold-lambda' :-(
-      `(funcall (identity (lambda (,@(mapcar #'car bindings))
-                            ,@(macroexp-unprogn body)))
+      `(funcall (lambda (,@(mapcar #'car bindings))
+                  ,@(macroexp-unprogn body))
                 ,@(mapcar #'cadr bindings)))
      ((null (cdr bindings))
       (macroexp-let* bindings body))