move backend functions to the bottom to suppress compiler warnings
Currently, Emacs byte-compiler warns bellow message.
$ emacs --batch -L . -f batch-byte-compile org-re-reveal.el
In org-re-reveal-define-backend:
org-re-reveal.el:78:13:Warning: reference to free variable
`org-re-reveal-keys`
I move backend functions to the bottom to suppress this compiler warning.