Item inner preludes

TL;DR : This allows this code with the new prelude keyword:

mod bar {
     #[derive(Default)]
     pub(crate) struct Bar(pub(crate) usize);
     #[derive(Default)]
     pub(crate) struct Baz(pub(crate) usize);
}
struct Foo {
    bar: Bar,
    prelude {
          use bar::*;
    },
    baz: Baz,
}

impl Foo {
    fn new() -> Self {
        Foo {
             bar: Default::default(),
             baz: Default::default(),
        }
    }
   fn into_baz(self) -> Baz {
        self.baz
    }
    prelude {
          use bar::*;
    }
}

Although the similar names, the motivation is quite different from RFC 890, in that this tries to address these issues:

• Allowing use items in more cases, bringing more ergonomics

Possible Future extensions:

• Allowing nested ADT definitions, in a way not affecting the typesystem.
• Generalize this to the mod levels, to actually cover the original custom prelude use case

Start with the proposed Rust examples and compare the motivation with RFC 890. Determine whether the item-level prelude behavior and its possible future extensions have enough design detail for an RFC; done means reaching a documented decision on the proposal, rather than implementing code in this repository.