This was discovered during a debugging session, and it only served to
mask the underlying error, which was not great.

* Document non-uniqueness of SetTopic IDs

* More comments on WithUniqueTopic

fix for:
```
error: using `clone` on type `[u8; 32]` which implements the `Copy`
trait
  --> xcm/xcm-builder/src/routing.rs:44:28
   |
44 |             message.0.push(SetTopic(unique_id.clone()));
   |                                     ^^^^^^^^^^^^^^^^^ help: try
removing the `clone` call: `unique_id`
   |
   = help: for further information visit
https://rust-lang.github.io/rust-clippy/master/index.html#clone_on_copy
   = note: `-D clippy::clone-on-copy` implied by `-D clippy::complexity`
```

* Tools for unique topic references

* Formatting

* Naming

* Repot into routing.rs.

* More things done

* Universal Exporter supports topic-as-reference

* Some tests for the topic routing

* More tests

* Paid bridge tests

* Add message ID to sending events

* Formatting

* fix and integrate into test nets

* Move DenyThenTry and friend from Cumulus

* Append SetTopic rather than prepend

* Docs

* Docs

* Work with new ProcessMessage ID API

* Formatting

* Fix build

* Fixes

* Formatting

* Update xcm/xcm-builder/src/barriers.rs

Co-authored-by: Francisco Aguirre <[email protected]>

* Update xcm/xcm-builder/src/routing.rs

Co-authored-by: Francisco Aguirre <[email protected]>

* Docs

* Rename message_hash

* Formatting

* ".git/.scripts/commands/fmt/fmt.sh"

* Rename

* Another Rename

* ".git/.scripts/commands/fmt/fmt.sh"

* ".git/.scripts/commands/fmt/fmt.sh"

* Update xcm/xcm-builder/src/routing.rs

Co-authored-by: Keith Yeung <[email protected]>

---------

Co-authored-by: Francisco Aguirre <[email protected]>
Co-authored-by: command-bot <>
Co-authored-by: Keith Yeung <[email protected]>