Moving kickout bucket selection to fastrandn

For faster initialization through avoiding mutex on global random.
Benchmark:

goos: linux
goarch: amd64
pkg: github.com/panmari/cuckoofilter
cpu: 12th Gen Intel(R) Core(TM) i7-1265U
                 │ master.bench │           fastrandn.bench           │
                 │    sec/op    │   sec/op     vs base                │
Filter_Insert-12    36.74n ± 3%   33.76n ± 1%  -8.11% (p=0.000 n=100)

cuckoofilter.go

@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"encoding/binary"
 	"fmt"
-	"math/rand"
 )
 
 // maxCuckooKickouts is the maximum number of times reinsert
@@ -85,7 +84,7 @@ func (cf *Filter) insert(fp fingerprint, i uint) bool {
 
 func (cf *Filter) reinsert(fp fingerprint, i uint) bool {
 	for k := 0; k < maxCuckooKickouts; k++ {
-		j := rand.Intn(bucketSize)
+		j := fastrandn(bucketSize)
 		// Swap fingerprint with bucket entry.
 		cf.buckets[i][j], fp = fp, cf.buckets[i][j]