godoc: link identifiers to declarations

The changes are almost completely self-contained
in the new file linkify.go. The other changes are
minimal and should not disturb the currently
working godoc, in anticipation of Go 1.1.

To disable the feature in case of problems, set
-links=false.

Fixes issue 2063.

R=adg, r
CC=golang-dev
https://codereview.appspot.com/7883044