consensus: Remove HEX_BUF_SIZE todo

Remove the todo, replace it with a comment stating that we guessed the
size and add an issue to measure and improve the value.

  https://github.com/rust-bitcoin/rust-bitcoin/issues/2391

bitcoin/src/consensus/serde.rs

@@ -66,7 +66,7 @@ pub mod hex {
         }
     }
 
-    // TODO measure various sizes and determine the best value
+    // We just guessed at a reasonably sane value.
     const HEX_BUF_SIZE: usize = 512;
 
     /// Hex byte encoder.