witx: Parse comments into documentation in AST

[pchickey]

Proof of concept to show that https://github.com/alexcrichton/wat/pull/26/ is a viable approach

[pchickey]

Todo:

• Only parse doc comments (;;; line comments or (;; block comments ;;)) into docs; ignore other comments.
• Trim whitespace
• Parse interior of doc comments as Markdown?

[alexcrichton]

👍

I whipped up a small render function which I think should do the trimming/doc comment parsing, and I think this looks like it might work well! I'll make a release of the wast crate with this support.

[pchickey]

I've got this PR to a point where I think it should be reviewed and merged:

• Doc comments are parsed at every node in the AST where they make sense to exist (imo). They are also output in the Render trait so that roundtrip testing still works. Unfortunately, whitespace doesn't roundtrip perfectly, and rather than come up with an elegant fix, I took the lazy way out and normalized all the whitespace in our witx files so that it will round trip.
• A very rudimentary markdown output is implemented. This will need some polishing before we can replace the currently hand-written docs with automated docs. I'll spend more time on that in the future but I also wanted to get multi-module support done this week, so I want to move on from this PR without finishing that aspect. However, I think it is good enough to merge this code, in particular because the next point causes merge conflicts with essentially everything:
• The existing comments in all our witx files have been converted to doc-comments. The sed script that does so is included in the commit message. A few places that mechanical conversion didn't work are fixed up manually.

[pchickey]

Merge conflicts resolved. After we do a snapshot, I'll resolve the conflicts one more time and then merge.

[pchickey]

Just rebased on #145 - will merge once that one is in

[sunfishcode]

LGTM!