auto merge of #12486 : MicahChalmer/rust/emacs-fixes-round-3, r=brson

auto merge of #12486 : MicahChalmer/rust/emacs-fixes-round-3, r=brson

I've added details in the description of each comment as to what it does, which I won't redundantly repeat here in the PR.  They all relate to indentation in the emacs rust-mode.

What I will note here is that this closes #8787.  It addresses the last remaining case (not in the original issue description but in a comment), of indenting `match` statements.  With the changes here, I believe every problem described in the issue description or comments of #8787 is addressed.