Support rust doc-comments - ///

[shanavas786]

/// gets expanded to // / where /// is expected

[davidshepherd7]

Fixing this is tricky because by the time the final / is inserted point is inside a comment, so the normal rules aren't applied.

A workaround is to do:

(setq electric-operator-enable-in-docs t)
(electric-operator-add-rules-for-mode 'text-mode (cons "///" " /// "))

but that has the downside that it will expand "///" to " /// " in all comments, strings and text based modes.

I'll carry on trying to think of a clean way to handle this...