Add tracing feature

[DaniPopes]

Add a "tracing" feature which imports the tracing crate, and instrument functions of interest with #[cfg_attr(feature = "tracing"), instrument)].

It has a real performance impact, so it should only be put scarcely

[gakonst]

Tracing has a comp time feature for removing the overhead https://docs.rs/tracing/latest/tracing/level_filters/index.html#compile-time-filters