fix: making invocation of tracing functions from Drop impls optional

[fogti]

They are now disabled by default and can be enabled using the feature flag tracing_in_drop if necessary.

Closes #231.

[taiki-e]

It is not clear if it is sufficient to disable it in the destructor of our type, since the problem appears to be happening in the destructor of thread-local storage.

Looking at the backtrace of #231 again, it appears that it is the tracing used in the polling::Poller::delete called in the destructor of calloop::sources::generic::Generic that is actually triggering the problem.

  20: tracing::span::Span::new
             at /home/sc/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/tracing-0.1.41/src/span.rs:437:9
  21: polling::epoll::Poller::delete
             at /home/sc/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/polling-3.7.4/src/epoll.rs:145:20
  22: polling::Poller::delete
             at /home/sc/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/polling-3.7.4/src/lib.rs:689:9
 23: <calloop::sources::generic::Generic<F,E> as core::ops::drop::Drop>::drop
             at /home/sc/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/calloop-0.13.0/src/sources/generic.rs:254:13

So, I think it would be more reliable to simply make tracing optional.

[fogti]

This does both.