Simplify Pin<T> to T

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

tests/rust/pin.rs

@@ -0,0 +1,8 @@
+#[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) {}

tests/rust/pin.toml

@@ -0,0 +1,21 @@
+header = """
+#if 0
+''' '
+#endif
+
+#ifdef __cplusplus
+template <typename T>
+using Pin = T;
+template <typename T>
+using Box = T*;
+#endif
+
+#if 0
+' '''
+#endif
+"""
+[export]
+exclude = [
+    "Pin",
+    "Box"
+]