Simplify Pin<T> to T

[kkoning]

cbindgen already simplifies MaybeUninit<T> and ManuallyDrop<T> to T. This adds Pin<T> as well.

[kkoning]

This is another one-liner, but this time treating Pin<T> the same way as MaybeUninit<T> and ManuallyDrop<T>, and with a new test.

Given this source:

#[repr(C)]
struct PinTest {
    pinned_box: Pin<Box<i32>>,
    pinned_ref: Pin<&mut i32>
}

#[no_mangle]
pub extern "C" fn root(s: Pin<&mut i32>, p: PinTest) {}

the binding created is:

typedef struct {
  int32_t *pinned_box;
  int32_t *pinned_ref;
} PinTest;

void root(int32_t *s, PinTest p);

[emilio]

Lgtm, thanks! The clippy lint failure is not your fault and should be fixed already in trunk.