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/matcher.go

@@ -131,7 +131,7 @@ func (m *Matcher) sliceChunks(chunks []*Chunk) [][]*Chunk {
 
 type partialResult struct {
 	index   int
-	matches []*Result
+	matches []Result
 }
 
 func (m *Matcher) scan(request MatchRequest) (*Merger, bool) {
@@ -162,7 +162,7 @@ func (m *Matcher) scan(request MatchRequest) (*Merger, bool) {
 		go func(idx int, slab *util.Slab, chunks []*Chunk) {
 			defer func() { waitGroup.Done() }()
 			count := 0
-			allMatches := make([][]*Result, len(chunks))
+			allMatches := make([][]Result, len(chunks))
 			for idx, chunk := range chunks {
 				matches := request.pattern.Match(chunk, slab)
 				allMatches[idx] = matches
@@ -172,7 +172,7 @@ func (m *Matcher) scan(request MatchRequest) (*Merger, bool) {
 				}
 				countChan <- len(matches)
 			}
-			sliceMatches := make([]*Result, 0, count)
+			sliceMatches := make([]Result, 0, count)
 			for _, matches := range allMatches {
 				sliceMatches = append(sliceMatches, matches...)
 			}
@@ -212,7 +212,7 @@ func (m *Matcher) scan(request MatchRequest) (*Merger, bool) {
 		}
 	}
 
-	partialResults := make([][]*Result, numSlices)
+	partialResults := make([][]Result, numSlices)
 	for _ = range slices {
 		partialResult := <-resultChan
 		partialResults[partialResult.index] = partialResult.matches