Provide C FFI types via core::ffi, not just in std

The ability to interoperate with C code via FFI is not limited to crates
using std; this allows using these types without std.

The existing types in `std::os::raw` become type aliases for the ones in
`core::ffi`. This uses type aliases rather than re-exports, to allow the
std types to remain stable while the core types are unstable.

This also moves the currently unstable `NonZero_` variants and
`c_size_t`/`c_ssize_t`/`c_ptrdiff_t` types to `core::ffi`, while leaving
them unstable.

library/std/src/lib.rs

@@ -261,6 +261,7 @@
 #![feature(const_socketaddr)]
 #![feature(const_trait_impl)]
 #![feature(container_error_extra)]
+#![feature(core_ffi_c)]
 #![feature(core_intrinsics)]
 #![feature(core_panic)]
 #![feature(custom_test_frameworks)]
@@ -315,6 +316,7 @@
 #![feature(prelude_import)]
 #![feature(ptr_as_uninit)]
 #![feature(ptr_internals)]
+#![feature(raw_os_nonzero)]
 #![feature(rustc_attrs)]
 #![feature(rustc_private)]
 #![feature(saturating_int_impl)]