Emacs: Update emacs mode to be more similar to the vim mode.

Copy the keyword list, and add `self` so that it is somewhat highlighted
(being liberal with the correct categories).
auto merge of #5103 : dbaupp/rust/emacs-mode-update, r=graydon

Copy the keyword list from rust.vim, and add `self` so that it is highlighted
(being liberal with the correct categories).

I'm not quite willing to dive in to clean up the emacs code yet, but at least this gets a (more) modern syntax highlighting list.