auto merge of #8031 : graydon/rust/emacs-mode-rewrite, r=catamorphism

The previous mode did a lot of very manual char-at-a-time parsing and was quite fragile and difficult to maintain. I talked to Marijn and he suggested I'd have better luck just rewriting it from scratch. This is the result of that effort; it seems to be faster, a bit easier on the eyes and more-or-less as well behaved when indenting. The algorithm is a bit different I suspect (it was hard to tell the previous one) but I tried to keep it simple and pleasant-looking.