Add AppendInt function with positive/negative number convertion to st…

…ring

Benchmark_ItoA/fiber             (pos_num)-12  190428255        6.13 ns/op       0 B/op       0 allocs/op
Benchmark_ItoA/fiber             (pos_num)-12  196626811        6.08 ns/op       0 B/op       0 allocs/op
Benchmark_ItoA/strconv.Itoa      (pos_num)-12   80716807       14.33 ns/op       4 B/op       1 allocs/op
Benchmark_ItoA/strconv.Itoa      (pos_num)-12   80445802       14.30 ns/op       4 B/op       1 allocs/op
Benchmark_ItoA/strconv.FormatInt_(pos_num)-12   81137728       14.30 ns/op       4 B/op       1 allocs/op
Benchmark_ItoA/strconv.FormatInt_(pos_num)-12   81345360       14.49 ns/op       4 B/op       1 allocs/op
Benchmark_ItoA/fiber             (neg_num)-12  151121002        7.81 ns/op       0 B/op       0 allocs/op
Benchmark_ItoA/fiber             (neg_num)-12  153566410        7.70 ns/op       0 B/op       0 allocs/op
Benchmark_ItoA/strconv.Itoa      (neg_num)-12   84505304       13.75 ns/op       5 B/op       1 allocs/op
Benchmark_ItoA/strconv.Itoa      (neg_num)-12   82524801       13.76 ns/op       5 B/op       1 allocs/op
Benchmark_ItoA/strconv.FormatInt (neg_num)-12   84884136       13.88 ns/op       5 B/op       1 allocs/op
Benchmark_ItoA/strconv.FormatInt (neg_num)-12   85829492       13.76 ns/op       5 B/op       1 allocs/op

README.md

@@ -17,7 +17,7 @@ pkg: github.com/gofiber/utils
 cpu: Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
 
 ```go
-// go test -benchmem -run=^$ -bench=Benchmark  -count=2
+// go test -benchmem -run=^$ -bench=Benchmark_  -count=2
 
 Benchmark_ToLowerBytes/fiber-12                29715831        36.44 ns/op       0 B/op       0 allocs/op
 Benchmark_ToLowerBytes/fiber-12                33316479        36.28 ns/op       0 B/op       0 allocs/op