MaybeUninit, ManuallyDrop, and Pin bindings is't transparent

Documentation for std types says what MaybeUninit<T>, ManuallyDrop<T>, and Pin<T> will be mapped into T, but instead of such behavior it creates additional structure.

Minimal reproducible example

lib.rs

use std::mem::ManuallyDrop;

#[unsafe(no_mangle)]
pub extern "C" fn test_func() -> ManuallyDrop<i32> {
    todo!()
}

cbindgen output (no additional flags and cbindgen.toml configs)

#include <cstdarg>
#include <cstdint>
#include <cstdlib>
#include <ostream>
#include <new>

template<typename T = void>
struct ManuallyDrop;

extern "C" {

ManuallyDrop<int32_t> test_func();

}  // extern "C"

Currently it's need to manually exclude the ManuallyDrop from binding generation, and write it by yourself. Here's example for it from unit tests.

I think it should generate such by default:

template <typename T>
using MaybeUninit = T;

Start with the std-types behavior described in docs.md and the reproduction in lib.rs. Compare the generated output with the existing tests/rust/maybeuninit.toml example, then trace how MaybeUninit, ManuallyDrop, and Pin are represented. Done means the documented transparent mapping is generated by default and the relevant tests cover it.