chore: Revert "use #[non_exhaustive] instead of private unit field" (#3323)

This reverts commit 575938d457.

examples/tinyhttp.rs

@@ -224,13 +224,12 @@ mod date {
 
     use time::{self, Duration};
 
-    #[non_exhaustive]
-    pub struct Now;
+    pub struct Now(());
 
     /// Returns a struct, which when formatted, renders an appropriate `Date`
     /// header value.
     pub fn now() -> Now {
-        Now
+        Now(())
     }
 
     // Gee Alex, doesn't this seem like premature optimization. Well you see