Problem: no benchmarks

Solution: add a single one

```sh
goos: linux
goarch: amd64
pkg: github.com/gomoni/it
cpu: AMD Ryzen 7 PRO 5850U with Radeon Graphics
BenchmarkRangeFilter
BenchmarkRangeFilter-16              448           2677041 ns/op
BenchmarkRangeFilter-16              435           2665242 ns/op
BenchmarkRangeFilter-16              442           2674867 ns/op
BenchmarkRangeFilter-16              432           2718882 ns/op
BenchmarkRangeFilter-16              408           2732315 ns/op
BenchmarkItFilterSlice
BenchmarkItFilterSlice-16             12          89357678 ns/op
BenchmarkItFilterSlice-16             12          89829099 ns/op
BenchmarkItFilterSlice-16             12          90112402 ns/op
BenchmarkItFilterSlice-16             12          89383558 ns/op
BenchmarkItFilterSlice-16             12          90150032 ns/op
BenchmarkItFilterFor
BenchmarkItFilterFor-16               19          58248848 ns/op
BenchmarkItFilterFor-16               19          58173282 ns/op
BenchmarkItFilterFor-16               18          58722532 ns/op
BenchmarkItFilterFor-16               19          58333300 ns/op
BenchmarkItFilterFor-16               19          58699434 ns/op
PASS
ok      github.com/gomoni/it    19.939s
michal@90michal:~/projects/gomoni/it>
```

bench_test.go

@@ -0,0 +1,49 @@
+package it_test
+
+import (
+	"testing"
+
+	"github.com/gomoni/it"
+)
+
+var in1M []int
+
+func init() {
+	in1M = make([]int, 1024*1024)
+	for idx := range in1M {
+		in1M[idx] = idx
+	}
+}
+
+func rangeFilter(in []int, filterFunc it.FilterFunc[int]) []int {
+	ret := make([]int, 0, len(in))
+	for _, i := range in {
+		if filterFunc(i) {
+			ret = append(ret, i)
+		}
+	}
+	return ret
+}
+
+func BenchmarkRangeFilter(b *testing.B) {
+	for range b.N {
+		rangeFilter(in1M, func(i int) bool { return i%2 == 0 })
+	}
+}
+
+func BenchmarkItFilterSlice(b *testing.B) {
+	for range b.N {
+		seq0 := it.Filter(it.FromSlice(in1M), func(i int) bool { return i%2 == 0 })
+		it.AsSlice(seq0)
+	}
+}
+
+func BenchmarkItFilterFor(b *testing.B) {
+	for range b.N {
+		seq0 := it.Filter(it.FromSlice(in1M), func(i int) bool { return i%2 == 0 })
+		cnt := 0
+		for value := range seq0 {
+			cnt += value
+		}
+	}
+}