auto merge of #19378 : japaric/rust/no-as-slice, r=alexcrichton

Now that we have an overloaded comparison (`==`) operator, and that `Vec`/`String` deref to `[T]`/`str` on method calls, many `as_slice()`/`as_mut_slice()`/`to_string()` calls have become redundant. This patch removes them. These were the most common patterns:

- `assert_eq(test_output.as_slice(), "ground truth")` -> `assert_eq(test_output, "ground truth")`
- `assert_eq(test_output, "ground truth".to_string())` -> `assert_eq(test_output, "ground truth")`
- `vec.as_mut_slice().sort()` -> `vec.sort()`
- `vec.as_slice().slice(from, to)` -> `vec.slice(from_to)`

---

Note that e.g. `a_string.push_str(b_string.as_slice())` has been left untouched in this PR, since we first need to settle down whether we want to favor the `&*b_string` or the `b_string[]` notation.

This is rebased on top of #19167

cc @alexcrichton @aturon

src/libstd/io/stdio.rs

@@ -528,7 +528,7 @@ mod tests {
             set_stdout(box w);
             println!("hello!");
         });
-        assert_eq!(r.read_to_string().unwrap(), "hello!\n".to_string());
+        assert_eq!(r.read_to_string().unwrap(), "hello!\n");
     }
 
     #[test]
@@ -543,6 +543,6 @@ mod tests {
             panic!("my special message");
         });
         let s = r.read_to_string().unwrap();
-        assert!(s.as_slice().contains("my special message"));
+        assert!(s.contains("my special message"));
     }
 }