Move IpAddr, SocketAddr and friends into core.

This is discussed in [RFC 2832], which is not accepted yet.

There seems to be no significant objection to including `IpAddr` and
friends. There is an objection to `SocketAddr` and friends:

> If the socketaddr types aren't identical across platforms, I don't
> think we want them in core.

Here is a possible interpretation:

core, and even the Rust language itself, already contains types with
platform-specific layouts. For example, `u64` has a platform-dependent
alignment, and that's visible even in otherwise fully encapsulating
types like `core::time::Duration`.

And core already has a `core::arch`, which is explicitly
architecture-dependent.

It seems the truly essential constraints of core are:

 - Nothing that does I/O. That goes in std.
 - No dynamic allocation. That goes in alloc.
 - No dependencies.

 - The public portable API feature set (eg. not `core::arch`) must be
   platform-independent. No unencapsulated `c_char` (signedness is
   platform-dependent) or `c_long` (range is platform-dependent).

   (Some types, like `c_schar`, could be debated, because one could
   argue that no serious platform would ever make `c_schar` different
   from `i8` ever again, but it isn't urgent to settle this.)

 - There must be fallback code so that core can be ported without any
   OS-specific work.

So I propose the path is: add the socket types to core, with
encapsulated OS-specific internal representations. The public-facing
feature set will be platform-independent. And add a `core::os` module,
similar to `core::arch` but for OS-specific features, which defines
`Ext` extension traits, similar to `std::os::*`, which expose the
platform-specific internal representations for FFI use cases.

A platform with no `core::os` support will still be able to use `core`
without any porting work, they just won't be able to access the
internal representations, which isn't a regression.

[RFC 2832]: rust-lang/rfcs#2832

Author: sunfishcode

library/core/src/lib.rs

@@ -116,6 +116,8 @@
 #![feature(const_int_unchecked_arith)]
 #![feature(const_intrinsic_copy)]
 #![feature(const_intrinsic_forget)]
+#![feature(const_ipv4)]
+#![feature(const_ipv6)]
 #![feature(const_likely)]
 #![feature(const_maybe_uninit_as_ptr)]
 #![feature(const_maybe_uninit_assume_init)]
@@ -132,6 +134,7 @@
 #![feature(const_size_of_val)]
 #![feature(const_slice_from_raw_parts)]
 #![feature(const_slice_ptr_len)]
+#![feature(const_socketaddr)]
 #![feature(const_str_from_utf8_unchecked_mut)]
 #![feature(const_swap)]
 #![feature(const_trait_impl)]
@@ -310,6 +313,7 @@ pub mod ffi;
 pub mod iter;
 #[unstable(feature = "once_cell", issue = "74465")]
 pub mod lazy;
+pub mod net;
 pub mod option;
 pub mod panic;
 pub mod panicking;