cross-referencing code & rfc: todo comments and html pages

- the rfc links back to the code now show any "todo" text that appears in the
  code. helps when looking at an rfc to find any work that may need to be done.
- html pages can now be generated to view code and rfc's side by side. clicking
  on links in one side opens the linked document in the other page, at the
  correct line number.

i'll be publishing the "dev" html version (latest commit on main branch) on the
mox website, updated with each commit. the dev pages will also link to the
latest released version.

rfc/Makefile

@@ -5,3 +5,6 @@ fetch:
 
 link:
 	go run -tags link link.go -- ../*.go ../*/*.go
+
+xr:
+	go run xr.go -- xr-dev $$(git tag | tail -n1) ../*.go ../*/*.go