Support raw pointers in cxx::bridge.

This allows raw pointers to types to be passed into and out of
cxx::bridge extern "C++" APIs. As normal with raw pointers in Rust,
there are no safety or lifetime guarantees.

Passing a raw pointer into such an API requires that the
function be marked "unsafe".
diff --git a/syntax/improper.rs b/syntax/improper.rs
index d672e7d..6a9b5ac 100644
--- a/syntax/improper.rs
+++ b/syntax/improper.rs
@@ -32,6 +32,7 @@
                 Definite(false)
             }
             Type::Ref(ty) => self.determine_improper_ctype(&ty.inner),
+            Type::Ptr(ty) => self.determine_improper_ctype(&ty.inner),
             Type::Array(ty) => self.determine_improper_ctype(&ty.inner),
         }
     }