Add #[non_exhaustive] to enums that might get new variants. (#673)

This just using them in patterns without a catchall. I left things alone
that seem very unlikely to change (`Value`, `ValueRef`, `DatabaseName`,
etc...). This might help reduce the number of breaking changes we need
(rusqlite is still pre-1.0 so it doesn't really matter that much, but
breaking changes complicate the story around when we can cut releases).

src/hooks.rs

@@ -12,6 +12,7 @@ use crate::{Connection, InnerConnection};
 /// `feature = "hooks"` Action Codes
 #[derive(Clone, Copy, Debug, PartialEq)]
 #[repr(i32)]
+#[non_exhaustive]
 pub enum Action {
     UNKNOWN = -1,
     SQLITE_DELETE = ffi::SQLITE_DELETE,