Don't clone when preparing Echo broadcast messages

[dvdplm]

Just a small optimization. Given that bincode does not encode struct names anyway, we can get away with serializing a raw Box and do the serialization work on references instead.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.06%. Comparing base (1d210d1) to head (9e38b90).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #143 +/- ## ======================================= Coverage 90.05% 90.06% ======================================= Files 43 43 Lines 7771 7768 -3 ======================================= - Hits 6998 6996 -2 + Misses 773 772 -1 ```

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