Switch from log to tracing and drop logger implementation

[morr0ne]

Origin should switch from the log crate to the tracing crate. Tracing brings support for scoping and nesting and overall has become the "standard" way to log/trace code. It's already used in official projects including rust itself.

Switching to tracing is extremely straightforward and fully backwards compatible since it can still emit log messages with a compatibility layer.

At the same time I'd like to propose a breaking change: the removal of env_logger and atomic-dbg-logger. It doesn't make any sense for a library to init the logger even a low level one as origin. The logging subscriber should be left to the consumers of the library instead. Currently it's very easy to just not enable the feature but I feel like they should just be removed for the next breaking release.

[sunfishcode]

Moving to tracing sounds good. What's the best thing to recommend for users wanting to just see the tracing output on stderr?

The motivation for initializing env_logger or atomic-dbg-logger is that origin can initialize them earlier in the program startup sequence than a user could, so they can issue messages for more of the things that origin does. I'm open to other approaches, but I'd like to retain that functionality.