Make cconv-analyse understand the need for closures.

Make cconv-analyse understand the need for closures.
* lisp/emacs-lisp/byte-lexbind.el (byte-compile-lforminfo-analyze):
Understand the :fun-body case for catch, save-window-excursion, and
condition-case.
(byte-compile-maybe-push-heap-environment): No need when nclosures is
zero and byte-compile-current-num-closures is -1.

* lisp/emacs-lisp/bytecomp.el (byte-compile-lambda): Fix `fun' that was not
renamed to `bytecomp-fun'.

* lisp/emacs-lisp/cconv.el (cconv-not-lexical-var-p): New function.
(cconv-freevars): Use it.
(cconv-closure-convert-rec): Avoid `position'.
(cconv-analyse-function): New function.
(cconv-analyse-form): Use it.  `inclosure' can't be nil any more.
Check lexical vars at let-binding time rather than when referenced.
For defuns to be in an empty environment and lambdas to take lexical args.
Pay attention to the need to build closures in catch, unwind-protect,
save-window-excursion, condition-case, and track-mouse.
Fix defconst/defvar handling.