Move BorrowedBuf and BorrowedCursor from std:io to core::io

Assigned new feature name `core_io_borrowed_buf` to distinguish from the
`Read::read_buf` functionality in `std::io`.

library/std/src/lib.rs

@@ -310,6 +310,7 @@
 // tidy-alphabetical-start
 #![feature(char_internals)]
 #![feature(core_intrinsics)]
+#![feature(core_io_borrowed_buf)]
 #![feature(duration_constants)]
 #![feature(error_generic_member_access)]
 #![feature(error_in_core)]