Refactor: inline core_io

* Extract the minimal set of definitions from `core_io` to `relibc` itself

* Remove dependency on `core_io`

src/platform/linux/mod.rs

@@ -1,5 +1,5 @@
+use crate::io::Write;
 use core::{arch::asm, ptr};
-use core_io::Write;
 
 use super::{errno, types::*, Pal};
 use crate::{

src/platform/redox/exec.rs

@@ -1,8 +1,8 @@
 use crate::{
     c_str::{CStr, CString},
-    core_io::{prelude::*, BufReader, SeekFrom},
     fs::File,
     header::string::strlen,
+    io::{prelude::*, BufReader, SeekFrom},
     platform::{
         sys::{S_ISGID, S_ISUID},
         types::*,