Optimise away functions in for-effect context

Optimise away functions in for-effect context

* lisp/emacs-lisp/byte-opt.el (byte-optimize-form-code-walker):
Turn functions into nil when compiled for-effect since they have no
side-effects on their own.  This may enable further improvements such
as the elimination of variable bindings.
`unwind-protect` forms can be treated as plain function call at this
point.  In particular, their unwind function argument should be
not optimised for effect since it's a function.