pkcs1v15: expose RsaSignatureAssociatedOid (#392)

This allows reuse of the `RsaSignatureAssociatedOid` trait to pull
implementation of `SignatureAlgorithmIdentifier` in other crates (like
`yubihsm.rs`).

src/pkcs1v15.rs

@@ -222,7 +222,7 @@ mod oid {
     use const_oid::ObjectIdentifier;
 
     /// A trait which associates an RSA-specific OID with a type.
-    pub(crate) trait RsaSignatureAssociatedOid {
+    pub trait RsaSignatureAssociatedOid {
         /// The OID associated with this type.
         const OID: ObjectIdentifier;
     }
@@ -258,6 +258,8 @@ mod oid {
     }
 }
 
+pub use oid::RsaSignatureAssociatedOid;
+
 #[cfg(test)]
 mod tests {
     use super::*;