1. Generate TOC for package pages using template,
instead of using JavaScript magic.  This makes the
pages generated by godoc -html easier to export
to other systems.

2. Make TOC one column.  It's hard to do two columns
portably without invoking JavaScript.

3. Since the TOC is only one column, show full type
signatures for functions and methods.  Many times
that's all you need to see anyway.

4. Name the section after the TOC "Overview".
Naming it something is important, to set it off
from the TOC and so that there's a quick link to
it in the TOC.

For now, some illustrative examples:

http://swtch.com:6060/pkg/io/
http://swtch.com:6060/pkg/strings/
http://swtch.com:6060/pkg/tabwriter/
http://swtch.com:6060/pkg/unicode/

Fixes #1982.

R=gri, bradfitz, r
CC=golang-dev
https://golang.org/cl/5303044