Move Opaque and Trivial marker types under a ::kind module
In the interest of keeping the root module focused on the most widely
used items only.
diff --git a/src/lib.rs b/src/lib.rs
index b53a7b6..815eacf 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -395,9 +395,7 @@
pub use crate::cxx_string::CxxString;
pub use crate::cxx_vector::CxxVector;
pub use crate::exception::Exception;
-pub use crate::extern_type::kind::Opaque;
-pub use crate::extern_type::kind::Trivial;
-pub use crate::extern_type::ExternType;
+pub use crate::extern_type::{kind, ExternType};
pub use crate::unique_ptr::UniquePtr;
pub use cxxbridge_macro::bridge;