FEAT: Use a separate union MaybeUninitCopy for ArrayString

This is the "real" union solution, and ArrayString can use it since its
backing array is Copy. Unfortunately, we'll have to use the Copy bound
on the type, making it "viral" and visible in the user API.

src/lib.rs

@@ -56,7 +56,10 @@ mod maybe_uninit;
 #[path="maybe_uninit_nodrop.rs"]
 mod maybe_uninit;
 
+mod maybe_uninit_copy;
+
 use maybe_uninit::MaybeUninit;
+use maybe_uninit_copy::MaybeUninitCopy;
 
 #[cfg(feature="serde-1")]
 use serde::{Serialize, Deserialize, Serializer, Deserializer};