Shorten the MaybeUninit Debug implementation

[tgross35]

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>,
}

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