Fix byte-compilation errors

[akirak]

Hello, I tried to build files in this repository and encountered some compilation errors.

• org-num.el seems to be a relatively new addition to org. It fails to build on Emacs 27.2, so I worked around it.
• Variables such as org-indent--deepest-level are unavailable until org-indent.el is loaded. The library should be explicitly loaded.
• You have to load org-element.el if you use functions such as org-element-map. They are not autoloaded.
• The trailing whitespace and misindentation are fixed. I'd suggest you turn on aggressive-indent-mode when you write elisp.

I also added the autoload magic to writer-mode, as the user don't have to load nano-writer.el until he/she uses the mode.

[rougier]

Oh nice, many thanks.