Expose calculate_token

Helps (or arguably fixes) scylladb#468

For smarter batch constitution (following up on scylladb#448), it is required
to have access to the token a particular statement would be directed to.
However, that is pretty difficult to do without access to
calculate_token.

That was initially put in scylladb#508, but planned to put in a separate pr to
keep it minimal
(scylladb#508 (comment))

It seems I had forgotten to open that separate PR, and I end up getting
bitten by that now that I want to constitute my batches in a smarter
way.

So here it is.

I'm only putting "helps" above because I think we may want to also
expose query planning (
`session.plan(prepared_statement, serialized_values) -> impl Iterator<Item = (Arc<Node>, ShardID)>`
) as that may make it significantly easier - but I'd like to keep this
PR that just *enables* the ideal behavior as simple as possible.

scylla/src/transport/session.rs

@@ -1747,7 +1747,7 @@ impl Session {
         .await
     }
 
-    fn calculate_token(
+    pub fn calculate_token(
         &self,
         prepared: &PreparedStatement,
         serialized_values: &SerializedValues,