(macroexp--with-extended-form-stack): Use plain `let`

(macroexp--with-extended-form-stack): Use plain `let`

`macroexp--with-extended-form-stack` used manual push/pop so that upon
non-local exits the "deeper" value is kept, so the error handler gets
to know what was the deeper value, so as to be able to compute more
precise error locations.
Replace this with a `handler-bind` which catches that "deeper" value
more explicitly.

* lisp/emacs-lisp/bytecomp.el (bytecomp--displaying-warnings):
Use `handler-bind` to catch the value of `byte-compile-form-stack`
at the time of the error.  Also consolidate the duplicated code.

* lisp/emacs-lisp/macroexp.el (macroexp--with-extended-form-stack):
Use a plain dynbound let-rebinding.