Implement conversion traits for IpAddr

[Yamakaky]

I'm thinking From<Ipv4Addr> and From<Ipv6Addr> at least.

[sfackler]

These seem pretty straightforward - feel free to open a PR adding them!

[seanmonstar]

I know I've often times wanted an easy way to create an SocketAddr without going through FromStr. There is a new constructor, but building an IpAddr is cumbersome. Maybe with From<[u8; 4]> or From<(u8, u8, u8, u8)>, or maybe as a new constructor on IpAddr.

I've wanted to reach for something like this, since it only requires the single SocketAddr import:

let addr = SocketAddr::new((127, 0, 0, 1), 8080);

But it would require changing SocketAddr::new to accept <T: Into<IpAddr>>...

[mattico]

A related (?) conversion which I think would be too confusing as a From impl would be converting IPv4 addresses to IPv6 mapped addresses (see https://tools.ietf.org/html/rfc4291 2.5.5.2). Perhaps .to_v6() or .to_mapped() or .to_ipv6_mapped_addr()? The shorter names may be confusing if you don't read the doc description, but so is most of the standard library.

[frewsxcv]

@mattico Interesting idea. It might be worth opening a separate issue for that so it can have its own separate discussion

[frewsxcv]

@mattico FYI, these methods already exist (I didn't know they existed until now):

• https://doc.rust-lang.org/std/net/struct.Ipv4Addr.html#method.to_ipv6_compatible
• https://doc.rust-lang.org/std/net/struct.Ipv4Addr.html#method.to_ipv6_mapped

Considering there are multiple ways to convert, adding a single From<IpV4> for IpV6 wouldn't be straightforward.

[Yamakaky]

Cool!