Implement Rust bindings for C++ operator[] overloads

[copybara-service]

Implement Rust bindings for C++ operator[] overloads

Instead of mapping directly to Rust's standard std::ops::Index and std::ops::IndexMut traits, this CL defines new Crubit-specific traits: CcIndex and CcIndexMut. This approach was chosen to address key interoperability challenges:

1. Return Types. C++ operator[] can return values, references, or proxy objects. Rust's Index traits are restrictive, requiring a reference (&Self::Output). The new traits use generic associated types (GATs) (type Output<'a> where Self: 'a;) to flexibly handle different return kinds and tie their lifetimes to the container.
2. !Unpin Containers. Many C++ types are not Unpin in Rust. std::ops::IndexMut takes &mut self, which is incompatible with !Unpin types that require Pin<&mut Self> for safe mutable access. CcIndexMut::cc_index_mut accepts self: Pin<&mut Self>.
3. !Unpin Items. The GAT-based Output type in CcIndexMut can return Pin<&mut Item> when necessary, preserving C++ safety invariants for non-movable items.

Integration tests in operators_index_test.rs cover combinations of Unpin / !Unpin containers and items.

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