p521: remove pub from arithmetic module

The relevant types from the `arithmetic` module are available via
re-exports, so direct access to the module is unnecessary. It was
accidentally exposed in the v0.13.1 release (#958).

Exposing the module directly is inconsistent with p192/p224/p256/p384,
so this removes it. This is technically a breaking change, but it was
added in v0.13.1 which was released a few minutes ago, and will be
yanked after this is released.

p521/src/lib.rs

@@ -16,7 +16,7 @@
 )]
 
 #[cfg(feature = "arithmetic")]
-pub mod arithmetic;
+mod arithmetic;
 
 #[cfg(feature = "ecdh")]
 pub mod ecdh;