tls12/tls13: make hash_algorithm crate internal

The `Tls12CipherSuite::hash_algorithm` and
`Tls13CipherSuite::hash_algorithm` functions were meant to be crate
internal, since their return type leaks the `ring::digest::Algorithm`
type. As written today these fns make updates to `*ring*` a breaking
change for the Rustls API.

This commit switches the visibility of both functions to be
crate-internal. Strictly speaking this is a breaking change, but we
don't expect there to be consumers of these functions and it unblocks
a *ring* update that would otherwise be breaking on its own.

rustls/src/tls12/mod.rs

@@ -171,7 +171,7 @@ impl Tls12CipherSuite {
     }
 
     /// Which hash function to use with this suite.
-    pub fn hash_algorithm(&self) -> &'static ring::digest::Algorithm {
+    pub(crate) fn hash_algorithm(&self) -> &'static ring::digest::Algorithm {
         self.hmac_algorithm.digest_algorithm()
     }
 }

rustls/src/tls13/mod.rs

@@ -76,7 +76,7 @@ pub struct Tls13CipherSuite {
 
 impl Tls13CipherSuite {
     /// Which hash function to use with this suite.
-    pub fn hash_algorithm(&self) -> &'static ring::digest::Algorithm {
+    pub(crate) fn hash_algorithm(&self) -> &'static ring::digest::Algorithm {
         self.hkdf_algorithm
             .hmac_algorithm()
             .digest_algorithm()