Move wrappers to their own module

In this commit, the module is kept private and its items
re-exported from the crate root. We name it `wrappers`,
but that's a bad (if accurate) name. We can bikeshed the
real name if/when we decide to make it pub.

Makes progress on #252

Author: joshlf

src/derive_util.rs

@@ -65,7 +65,7 @@ macro_rules! union_has_padding {
 
 #[cfg(test)]
 mod tests {
-    use crate::testutil::*;
+    use crate::util::testutil::*;
 
     #[test]
     fn test_struct_has_padding() {