Emit const __BindgenUnionField accessors and comply with Rust 2024 unsafe-op-in-unsafe-fn

[mematthias]

This PR adjusts the generated __BindgenUnionField implementation to be clearer about unsafety and compatible with the Rust 2024 unsafe-op-in-unsafe-fn rules: https://doc.rust-lang.org/edition-guide/rust-2024/unsafe-op-in-unsafe-fn.html

In Rust 2024, unsafe operations inside an unsafe fn must be wrapped in an explicit unsafe {} block. Instead of relying on BindgenContext::wrap_unsafe_ops, the generated code now directly wraps the mem::transmute(self) call in unsafe { ... }, making the unsafety explicit at the operation site.

Additionally, the generated methods are now const:

• new is emitted as pub const fn
• as_ref / as_mut are emitted as pub const unsafe fn

This is possible because mem::transmute is available in const contexts since Rust 1.56. The change simplifies the output and improves forward compatibility without changing the observable behavior of the bindings.