struct HandleLayoutCheck

Handles are constructed and destroyed by Rust (rust_handle_make, rust_handle_dup,

rust_handle_destroy), so every Handle the C++ accessors in handle.h touch was laid out by the

Rust `Handle` in //zircon/kernel/object/handle.rs, which pins its own layout with a matching set

of assertions. The assertions below pin the C++ view of the very same bytes, so that the two

can never drift apart silently. They live here rather than in the header because offsetof() on

Handle's private members needs friend access.