Fix clippy::used_underscore_items warning

```
error: used underscore-prefixed item
  --> src/io/mod.rs:24:5
   |
24 |     _assert_object_safety::<dyn Read>();
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: item is defined here
  --> src/io/mod.rs:23:5
   |
23 |     fn _assert_object_safety<T: ?Sized>() {}
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#used_underscore_items
   = note: `-D clippy::used-underscore-items` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::used_underscore_items)]`

error: used underscore-prefixed item
  --> src/io/mod.rs:25:5
   |
25 |     _assert_object_safety::<dyn Write>();
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: item is defined here
  --> src/io/mod.rs:23:5
   |
23 |     fn _assert_object_safety<T: ?Sized>() {}
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#used_underscore_items

error: used underscore-prefixed item
  --> src/io/mod.rs:26:5
   |
26 |     _assert_object_safety::<dyn Seek>();
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: item is defined here
  --> src/io/mod.rs:23:5
   |
23 |     fn _assert_object_safety<T: ?Sized>() {}
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#used_underscore_items
```

src/io/mod.rs

@@ -20,10 +20,10 @@ mod stdio;
 use core::fmt;
 
 const _: fn() = || {
-    fn _assert_object_safety<T: ?Sized>() {}
-    _assert_object_safety::<dyn Read>();
-    _assert_object_safety::<dyn Write>();
-    _assert_object_safety::<dyn Seek>();
+    fn assert_object_safety<T: ?Sized>() {}
+    assert_object_safety::<dyn Read>();
+    assert_object_safety::<dyn Write>();
+    assert_object_safety::<dyn Seek>();
 };
 
 pub(crate) fn default_read_exact<R: ?Sized + Read>(this: &mut R, mut buf: &mut [u8]) -> Result<()> {