Support sub benchmarks

[miry]

func BenchmarkMerge(b *testing.B) {
  b.Run("hash", func(b *testing.B) {
  }
  b.Run("array", func(b *testing.B) {
  }
}

Would generate output:

BenchmarkMerge/hash-4
BenchmarkMerge/array-4

Add in the list of valid characters / resolve problem.

Sample charts: http://benchgraph.codingberg.com/45

func BenchmarkAccessStructure(b *testing.B) {
	for _, size := range []int{1, 10, 100, 1000, 10000, 100000} {
		benchmarkAccessStructure(b, size)
	}
}

func benchmarkAccessStructure(b *testing.B, size int) {
	var indexes = make([]int, size, size)
	var arr = make([]int, size, size)
	var hash = make(map[int]int)

	rand.Seed(42)
	for i := 0; i < size; i++ {
		indexes[i] = rand.Intn(size)
		arr[i] = i
		hash[i] = i
	}

	b.ResetTimer()

	b.Run(fmt.Sprintf("Array_%d", size), func(b *testing.B) {
		for i := 0; i < b.N; i++ {
			indx := indexes[i%size] % size
			_ = arr[indx]
		}
	})

	b.Run(fmt.Sprintf("Hash_%d", size), func(b *testing.B) {
		for i := 0; i < b.N; i++ {
			indx := indexes[i%size] % size
			_ = hash[indx]
		}
	})
}