Use *_test packages to make examples copy-pasteable (#124)

The main reason for this PR was to make the examples in documentation
copy-pasteable, but there are some additional benefits too.

For example, the pool example before this change:

```go
p := New().WithMaxGoroutines(3)
for i := 0; i < 5; i++ {
	p.Go(func() {
		fmt.Println("conc")
	})
}
p.Wait()
```

and after:

```go
p := pool.New().WithMaxGoroutines(3) // has package name
for i := 0; i < 5; i++ {
	p.Go(func() {
		fmt.Println("conc")
	})
}
p.Wait()
```