Make `PamHandle` and `PamModuleHandle` traits. This creates traits for PAM functionality and pulls the definitions of that functionality out of the original `PamHandle` (renamed to `LibPamHandle`) and into those traits. This supports testing PAM module implementations using mock PAM library implementations. Also uses a better representation of opaque pointers.

--- a/src/lib.rs	Thu May 22 02:08:10 2025 -0400
+++ b/src/lib.rs	Tue May 27 14:37:28 2025 -0400
@@ -35,6 +35,6 @@
 #[doc(inline)]
 pub use crate::{
     constants::{ErrorCode, Flags, Result},
-    handle::PamHandle,
+    handle::{LibPamHandle, PamHandle, PamModuleHandle},
     module::PamModule,
 };