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/libstd/dynamic_lib.rs

@@ -22,7 +22,6 @@ use mem;
 use env;
 use str;
 
-#[allow(missing_copy_implementations)]
 pub struct DynamicLibrary {
     handle: *mut u8
 }