Switch missing_copy_implementations to default-allow

This was particularly helpful in the time just after OIBIT's
implementation to make sure things that were supposed to be Copy
continued to be, but it's now creates a lot of noise for types that
intentionally don't want to be Copy.

src/libcore/str/mod.rs

@@ -149,7 +149,6 @@ impl FromStr for bool {
 
 /// An error returned when parsing a `bool` from a string fails.
 #[derive(Debug, Clone, PartialEq)]
-#[allow(missing_copy_implementations)]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct ParseBoolError { _priv: () }