Use dyn for a dynamic trait

[linkmauve]

Muxer is a trait, and passing dynamic traits without the dyn keyword is now deprecated.

[coveralls]

Coverage remained the same at 83.177% when pulling a51c710 on linkmauve:patch-1 into 12f041f on xiph:master.

[lu-zero]

Thank you for spotting it :)

I checked with #![deny(bare_trait_objects)] that the rest of the codebase is clean.

[linkmauve]

Maybe we should add this deny crate-wide?

[lu-zero]

Could be a good idea let see what the team thinks :)

[shssoichiro]

It will at the least be warn by default in an upcoming rustc (slated for 1.37): rust-lang/rust#61203. But we may want to change it before then.