Customise Display impl of ContactId

This brings the Display of ContactId in line with those of ChatId etc,
which is a bit clearer is logs and such places.

It also updates an SQL query to rely on the ToSql impl of ContactId
rather than it's Display when building the query.

src/sql.rs

@@ -797,6 +797,19 @@ async fn prune_tombstones(sql: &Sql) -> Result<()> {
     Ok(())
 }
 
+/// Helper function to return comma-separated sequence of `?` chars.
+///
+/// Use this together with [`rusqlite::ParamsFromIter`] to use dynamically generated
+/// parameter lists.
+pub fn repeat_vars(count: usize) -> Result<String> {
+    if count == 0 {
+        bail!("Must have at least one repeat variable");
+    }
+    let mut s = "?,".repeat(count);
+    s.pop(); // Remove trailing comma
+    Ok(s)
+}
+
 #[cfg(test)]
 mod tests {
     use async_std::channel;