Rollup merge of #109174 - soerenmeier:cursor_fns, r=dtolnay

Replace `io::Cursor::{remaining_slice, is_empty}`

This is a late follow up to the concerns raised in https://github.com/rust-lang/rust/issues/86369.

https://github.com/rust-lang/rust/issues/86369#issuecomment-953096691
> This API seems focussed on the `Read` side of things. When `Seek`ing around and `Write`ing data, `is_empty` becomes confusing and `remaining_slice` is not very useful. When writing, the part of the slice before the cursor is much more interesting. Maybe we should have functions for both? Or a single function that returns both slices? (If we also have a `mut` version, a single function would be useful to allow mutable access to both sides at once.)

New feature name: `cursor_remaining` > `cursor_split`.
Added functions:
```rust
fn split(&self) -> (&[u8], &[u8]);
// fn before(&self) -> &[u8];
// fn after(&self) -> &[u8];
fn split_mut(&mut self) -> (&mut [u8], &mut [u8]);
// fn before_mut(&mut self) -> &mut [u8];
// fn after_mut(&mut self) -> &mut [u8];
```

A question was raised in https://github.com/rust-lang/rust/issues/86369#issuecomment-927124211 about whether to return a lifetime that would reflect the lifetime of the underlying bytes (`impl Cursor<&'a [u8]> { fn after(&self) -> &'a [u8] }`). The downside of doing this would be that it would not be possible to implement these functions generically over `T: AsRef<[u8]>`.

## Update
Based on the review, before* and after* methods where removed.

library/std/src/io/tests.rs

@@ -676,13 +676,13 @@ fn cursor_read_exact_eof() {
 
     let mut r = slice.clone();
     assert!(r.read_exact(&mut [0; 10]).is_err());
-    assert!(r.is_empty());
+    assert!(Cursor::split(&r).1.is_empty());
 
     let mut r = slice;
     let buf = &mut [0; 10];
     let mut buf = BorrowedBuf::from(buf.as_mut_slice());
     assert!(r.read_buf_exact(buf.unfilled()).is_err());
-    assert!(r.is_empty());
+    assert!(Cursor::split(&r).1.is_empty());
     assert_eq!(buf.filled(), b"123456");
 }