In the process, rewrite index.go to use slices instead
of vectors, rewrite for-loops into range loops, and
generally simplify code (this code was written before
the launch of go and showed its age).

Also, fix a wrong import in appinit.go.

No significant performance changes (improvements);
most of time is spent elsewhere (measured on an stand-
alone MacBook Pro with SSD disk, running standard
godoc settings: godoc -v -http=:7777 -index).

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4875056