godoc: provide mode which shows exported interface in "source form"

- on the commandline: godoc -x big
- in a webpage: provide form parameter ?m=src

Known issues:
- Positioning of comments incorrect in several cases. Separate CL.
- Need a link/menu to switch between different modes of presentation
  in the web view.

R=rsc
CC=golang-dev
https://golang.org/cl/376041