How to simplify struct mutating in Swift

Issue #354

In Construction, we have a build method to apply closure to inout struct.

We can explicitly define that with withValue

func withValue<T>(_ value: T, closure: (inout T) -> Void) -> T {
    var mutableValue = value
    closure(&mutableValue)
    return mutableValue
}

So we can modify Protobuf structs easily

user.book = withValue(Book()) {
    $0.price = 300
    $0.author = withValue(Author()) {
        $0.name = "Thor"
    }
}