fix(exp): set capacity for each batch (#490)

Inspired by the new golang 1.23 Chunk implementation:
https://togithub.com/golang/go/issues/53987

hcloud/exp/kit/sliceutil/slice.go

@@ -2,8 +2,11 @@ package sliceutil
 
 // Batches splits a slice into multiple batches of a desired size.
 func Batches[T any](all []T, size int) (batches [][]T) {
+	batches = make([][]T, 0, (len(all)/size)+1)
 	for size < len(all) {
-		all, batches = all[size:], append(batches, all[:size])
+		// Set the capacity of each chunk so that appending to a chunk does not
+		// modify the original slice.
+		all, batches = all[size:], append(batches, all[:size:size])
 	}
 	return append(batches, all)
 }

hcloud/exp/kit/sliceutil/slice_test.go

@@ -11,7 +11,13 @@ func TestBatches(t *testing.T) {
 	batches := Batches(all, 2)
 
 	assert.Len(t, batches, 3)
+
 	assert.Equal(t, []int{1, 2}, batches[0])
+	assert.Equal(t, 2, cap(batches[0]))
+
 	assert.Equal(t, []int{3, 4}, batches[1])
+	assert.Equal(t, 2, cap(batches[1]))
+
 	assert.Equal(t, []int{5}, batches[2])
+	assert.Equal(t, 1, cap(batches[2]))
 }