Auto merge of #131715 - tgross35:add-const_sockaddr_setters, r=Amanieu
Add an unstable `const_sockaddr_setters` feature
Unstably add `const` to the `sockaddr_setters` methods. Included API:
```rust
// core::net
impl SocketAddr {
pub const fn set_ip(&mut self, new_ip: IpAddr);
pub const fn set_port(&mut self, new_port: u16);
}
impl SocketAddrV4 {
pub const fn set_ip(&mut self, new_ip: Ipv4Addr);
pub const fn set_port(&mut self, new_port: u16);
}
impl SocketAddrV6 {
pub const fn set_ip(&mut self, new_ip: Ipv6Addr);
pub const fn set_port(&mut self, new_port: u16);
}
```
Tracking issue: <https://github.com/rust-lang/rust/issues/131714>
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