Initial port to tracing

[lu-zero]

It behaves as fern since no changes to the logging are present, but should let you use the tracing api if you need something more refined.

It clashes with the tracing feature that enables hawktracer and that's in itself unfortunate.

[coveralls]

Coverage decreased (-0.02%) to 83.094% when pulling c77127b70f8d61ef2346d0a6adfdb2a8102e4638 on rust-av:tracing into 26fa9edb4b31c5790ee69911203e2e8a9e7d54e7 on xiph:master.

[lu-zero]

Done in 1a774396a70b9003f0fec1eabd04de345349ea32