Add From<[u8; 4]> for Ipv4Addr

[xyzzyz]

Currently, the only way to create server_name::Ipv4Addr in no_std world is to parse it from string. This is somewhat suboptimal. This PR adds impl From<[u8; 4]> for Ipv4Addr, to parallel the already existing impl for Ipv6Addr.

[djc]

Is there a similar impl in core?

[ctz]

(An alternative to this, if you desire, is Ipv4Addr::new which would be const -- https://doc.rust-lang.org/std/net/struct.Ipv4Addr.html#method.new)

[ctz]

Is there a similar impl in core?

https://doc.rust-lang.org/std/net/struct.Ipv4Addr.html#impl-From%3C%5Bu8;+4%5D%3E-for-Ipv4Addr

[xyzzyz]

@ctz unfortunately, without std feature, there's no way to convert core::net::Ipv4Addr to rustls_pki_types::Ipv4Addr, hence my PR.

I guess one could replace the existing impl From<std::net::Ipv4Addr> for rustls_pki_types::Ipv4Addr that's feature-gated behind std feature with impl From<core::net::Ipv4Addr> for rustls_pki_types::Ipv4Addr and remove feature gating, WDYT?