Create layout-compatible standard lib types in Rust and C++

[teor2345]

Using layout-compatible types means simple fields can be access directly from a foreign language, improving performance.

C++ Language or Library Changes

Add a function to get the allocation metadata given a new’d pointer, like I can get from delete.
This is useful for custom allocators.

Existing Work

Crubit currently does this for:

• C++ (string_view, span, optional, vector, unique_ptr, shared_ptr)
• Rust (everything non-generic type plus Option + Result + slices)