Use gobs to serialize indexes instead of encoding/binary.

Even with gobs, serialize data in slices instead of
applying gob to the entire data structure at once,
to reduce the amount of extra buffer memory needed
inside gob.

7x faster Write/Read for new BenchmarkSaveRestore
compared to old code; possibly because encoding/binary
is more expensive for int32 slice elements (interface
call to get little/big endian encoding), while gob's
encoding is fixed (unconfirmed).

new (using gobs):
suffixarray.BenchmarkSaveRestore	       1	2153604000 ns/op

old (using encoding/binary):
suffixarray.BenchmarkSaveRestore	       1	15118322000 ns/op

The actual serialized data is slightly larger then using
the old code for very large indices because full 32bit indices
require 5bytes using gobs instead of 4bytes (encoding/binary)
in serialized form.

R=r
CC=golang-dev
https://golang.org/cl/5087041