Add a convenience function "replace-hook."

The hook mechanism requires adding function names (i.e., symbols) to a
hook; a hook is added only if it is not present before ("add-hook"). To
make it easy to, for example, rebind the modifier from :mod-1 (the "Alt"
key) to :mod (the "Win" key), we need an easy way to add/update a binding
to *binding-hook*.

A use case in "~/.clfswmrc" to make the aforesaid changes:

(replace-hook *binding-hook*
	      my-change-default-modifier
	      #'(lambda () (binding-substitute-modifier :mod-4 :mod-1)))