Create a simpler way to delete address

[acst1223]

First I want to thanks for the work to this library! It has been really helpful to me.

Recently when I tried to delete an address with this library, I found it was much more complex compared to adding an address. When adding an address we have dedicated add() function in AddressHandle, which accepts the index, address, and prefix_len directly, but when deleting, the del() function we can use is too simple: we need to construct the AddressMessage on our own. There is even no example for that and we already have some issues for this: #80, #100.

I have no idea why del() is made this way. I think it's not too much work to add another function like del_addr_prefix_len() to delete an address as easy as adding them with add(), and also to add an example for it. I can implement it if this is accepted.

[cathay4t]

I am not sure linux kernel has ability of delete IP address matches with specified attributes. Anyway, yes, del_addr_prefix_len() make sense.

Meanwhile, regarding complexity on creating AddressMessage, could you create something similar to LinkMessageBuilder<T>::build()? Something like AddressMessageBuilder<Ipv4Addr>::new().del(addr).build() -> AddressMessage.

[acst1223]

Thanks for the prompt response! I think I can start with implementing the AddressMessageBuilder<T> which can make life easier.