sync: Add a method to WaitGroup to simplify Add, Done, go func()

[darkfeline]

When using sync.WaitGroup, very often it is used like so:

var wg sync.WaitGroup
wg.Add(1)
go func() {
    defer wg.Done()
    // Some code
}()

Of course, this is not the only use case, but it is by far the most common. It would be convenient to add a method used like:

var wg sync.WaitGroup
wg.Run(func() {
    // Some code
})

func (wg *sync.WaitGroup) Run(f func()) {
    wg.Add(1)
    go func() {
        defer wg.Done()
        f()
    }()
}

[seankhliao]

Duplicate of #39863