CL 5040041 (https://golang.org/cl/5040041)
changed the use of []int to []int32 internally so
that encoding/binary could be used. This is no
longer needed (gobs can encode ints), and using
[]int is more in sync w/ the semantics of the data
structure (the index elements are indices which are
ints). Changing it back.

R=r
CC=golang-dev
https://golang.org/cl/5141049