Rollup merge of rust-lang#94503 - joshtriplett:core-ffi-c, r=Amanieu

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

Tracking issue: rust-lang#94501

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.

Historically, we didn't do this because these types are target-dependent.
However, `core` itself is also target-dependent. `core` should not call
any OS services, but it knows the target and the target's ABI.

Author: matthiaskrgr

library/std/src/os/raw/char.md renamed to library/core/src/ffi/c_char.md

@@ -2,8 +2,7 @@ Equivalent to C's `char` type.
 
 [C's `char` type] is completely unlike [Rust's `char` type]; while Rust's type represents a unicode scalar value, C's `char` type is just an ordinary integer. On modern architectures this type will always be either [`i8`] or [`u8`], as they use byte-addresses memory with 8-bit bytes.
 
-C chars are most commonly used to make C strings. Unlike Rust, where the length of a string is included alongside the string, C strings mark the end of a string with the character `'\0'`. See [`CStr`] for more information.
+C chars are most commonly used to make C strings. Unlike Rust, where the length of a string is included alongside the string, C strings mark the end of a string with the character `'\0'`. See `CStr` for more information.
 
 [C's `char` type]: https://en.wikipedia.org/wiki/C_data_types#Basic_types
 [Rust's `char` type]: char
-[`CStr`]: crate::ffi::CStr