Paragraph filling, more indentation fixes, and tests for the emacs mode

Here are fixes for more problems mentioned in #8787.  I think I've addressed everything mentioned there except for @nikomatsakis's comment about match/patterns now.  (This also fixes the bug in struct alignment that @pnkfelix mentioned from my earlier pull request #8872.)

The biggest change here is to make fill-paragraph (M-q) and auto-fill-mode work inside different variations of multi-line and doc comments.  Because of the way emacs paragraph fills work (callbacks interacting with global regexp variables that are used in odd ways) there were quite a few edge cases that I had to work around.

The only way I was able to keep it all straight was to create some regression tests.  They use the emacs lisp regression testing tool ERT, and are included as the last commit here.  I added a few tests for indentation as well.  I have not attempted to integrate the tests into the overall rust compiler build process, since I can't imagine anyone would want the compiler build to have a dependency on emacs.  Maybe at some point tools like this get their own repositories?  Just a thought.

One other thought related to the tests: should there be a place to put these types of style samples that isn't specific to one text editor?  Maybe as part of an official rust style guide, but in a form that would allow tools like this to pull out the samples and use them for tests?