A MsgId newtype

This more strongly types the ubiquitous message id type by no longer
making it an integer.  It keeps the actual ID opaque.  Only for the
generic job API the number keeps being used.  Some locations also need
to create it from an integer and call MsgId::new().

src/blob.rs

@@ -193,6 +193,8 @@ impl<'a> BlobObject<'a> {
     /// the database.  Do not use this unless you're about to store
     /// this string in the database or [Params].  Eventually even
     /// those conversions should be handled by the type system.
+    ///
+    /// [Params]: crate::param::Params
     pub fn as_name(&self) -> &str {
         &self.name
     }