Consolidate Result and rank structs

By not storing item index twice, we can cut down the size of Result
struct and now it makes more sense to store and pass Results by values.
Benchmarks show no degradation of performance by additional pointer
indirection for looking up index.

src/cache_test.go

@@ -7,8 +7,8 @@ func TestChunkCache(t *testing.T) {
 	chunk2 := make(Chunk, chunkSize)
 	chunk1p := &Chunk{}
 	chunk2p := &chunk2
-	items1 := []*Result{&Result{}}
-	items2 := []*Result{&Result{}, &Result{}}
+	items1 := []Result{Result{}}
+	items2 := []Result{Result{}, Result{}}
 	cache.Add(chunk1p, "foo", items1)
 	cache.Add(chunk2p, "foo", items1)
 	cache.Add(chunk2p, "bar", items2)