CellID.AllNeighbors: Allocate memory in one go (#213)

Pre-allocate memory to avoid triggering expansion, and enhance performance.

- Before：
```
BenchmarkAllNeighbors/level12         	 3699290	       322.9 ns/op	     120 B/op	       4 allocs/op
BenchmarkAllNeighbors/level14         	 1699875	       700.6 ns/op	     504 B/op	       6 allocs/op
BenchmarkAllNeighbors/level16         	  594056	        2091 ns/op	    2040 B/op	       8 allocs/op
```
- After：
```
BenchmarkAllNeighbors/level12         	 5084275	       254.1 ns/op	      64 B/op	       1 allocs/op
BenchmarkAllNeighbors/level14         	 2295961	       510.6 ns/op	     160 B/op	       1 allocs/op
BenchmarkAllNeighbors/level16        	  786132	        1610 ns/op	     576 B/op	       1 allocs/op
```

Author: fx408

s2/cellid.go

@@ -276,7 +276,7 @@ func (ci CellID) AllNeighbors(level int) []CellID {
 		return nil
 	}
 
-	var neighbors []CellID
+	var neighbors = make([]CellID, 0, (4<<(level-ci.Level()))+4)
 
 	face, i, j, _ := ci.faceIJOrientation()
 

s2/cellid_test.go

@@ -15,6 +15,7 @@
 package s2
 
 import (
+	"fmt"
 	"math"
 	"reflect"
 	"testing"
@@ -1046,6 +1047,25 @@ func TestCellIDCenterFaceSiTi(t *testing.T) {
 	}
 }
 
+func BenchmarkAllNeighbors(b *testing.B) {
+	level := 12
+	ll := LatLngFromDegrees(10.100001, 10.100002)
+	cellID := CellIDFromLatLng(ll).Parent(level)
+
+	cases := []int{level, level + 2, level + 4}
+
+	for _, n := range cases {
+		b.Run(fmt.Sprintf("level%d", n), func(b *testing.B) {
+			b.ResetTimer()
+			b.ReportAllocs()
+
+			for i := 0; i < b.N; i++ {
+				cellID.AllNeighbors(n)
+			}
+		})
+	}
+}
+
 // TODO(roberts): Remaining tests to convert.
 // Coverage
 // TraversalOrder