godoc: use new index/suffixarray serialization code

godoc: use new index/suffixarray serialization code

When saving/restoring the fulltext index, the entire
respective suffixarray is now saved/restored (as opposed
to the indexed data only, and the suffixarray recreated).
This saves significant start-up time for large indexes,
at the cost of significantly larger index files.

R=r
CC=golang-dev
https://golang.org/cl/5037043