Shorten the MaybeUninit Debug implementation

Currently the `Debug` implementation for `MaybeUninit` winds up being
pretty verbose. This struct:

    #[derive(Debug)]
    pub struct Foo {
        pub a: u32,
        pub b: &'static str,
        pub c: MaybeUninit<u32>,
        pub d: MaybeUninit<String>,
    }

Prints as:

    Foo {
        a: 0,
        b: "hello",
        c: core::mem::maybe_uninit::MaybeUninit<u32>,
        d: core::mem::maybe_uninit::MaybeUninit<alloc::string::String>,
    }

The goal is just to be a standin for content so the path prefix doesn't
add any useful information. Change the implementation to trim
`MaybeUninit`'s leading path, meaning the new result is now:

    Foo {
        a: 0,
        b: "hello",
        c: MaybeUninit<u32>,
        d: MaybeUninit<alloc::string::String>,
    }

library/core/src/mem/maybe_uninit.rs

@@ -255,7 +255,10 @@ impl<T: Copy> Clone for MaybeUninit<T> {
 #[stable(feature = "maybe_uninit_debug", since = "1.41.0")]
 impl<T> fmt::Debug for MaybeUninit<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.pad(type_name::<Self>())
+        let full_name = type_name::<Self>();
+        // NB: this needs to be changed if `MaybeUninit` moves modules
+        let short_name = full_name.split_once("mem::maybe_uninit::").unwrap().1;
+        f.pad(short_name)
     }
 }