libstd.natvis

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    std::collection::Hash* container visualizers

    Current std impls:
      std::collections::hash::set::HashSet<K, S>      is implemented in terms of...
      hashbrown::set::HashSet<K, S>                   is implemented in terms of...
      hashbrown::map::HashMap<K, V, S>                is implemented in terms of...
      hashbrown::raw::RawTable<(K, V)>

    Ideally, we'd teach rustc to scan dependencies/crates for .natvis files so
    the bulk of this could live alongside the hashbrown crate implementation,
    and std would just forward using e.g. <ExpandedItem>base</ExpandedItem>.

    However, Given that std...Hash*Set* is currently implemented in terms of
    hashbrown...Hash*Map*, which would visualize poorly, we want to customize the
    look/feel at the std type level *anyways*...

    References:
      https://github.com/rust-lang/rust/blob/master/src/libstd/collections/hash/map.rs
      https://github.com/rust-lang/rust/blob/master/src/libstd/collections/hash/set.rs
      https://github.com/rust-lang/hashbrown/blob/master/src/map.rs
      https://github.com/rust-lang/hashbrown/blob/master/src/set.rs
      https://github.com/rust-lang/hashbrown/blob/master/src/raw/mod.rs
  -->

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