Restructure initialization setup

[sfackler]

try_set_logger_raw is now set_logger and try_set_logger is now
set_boxed_logger. The use_std feature is now disabled by default. The
old set_logger has been removed.

When we did the crate evaluation for log, we wanted to add more variants
of set_logger that e.g. panicked by default, but I'm no longer convinced
that's a good idea. There are going to be very few instances of actually
calling these methods explicitly, since each logger implementation
should be providing their own init method that calls them. Having a huge
constellation of functions that all do basically the same thing just
makes things really confusing. We also don't want to encourage logger
implementations to only provide an init function that panics because a
common way of working with logging in tests is to try to init the system
in each test and ignore the result.

r? @alexcrichton

[sfackler]

Both this example and the one below were ignoring the result of try_set_logger....

[alexcrichton]

Sounds great to me!

[meh]

What's the rationale behind disabling use_std by default?

Took me a while to find I had to enable something to get set_boxed_logger since I just generated the docs and didn't see anything for that.

[sfackler]

The vast majority of consumers of log don't need to use it, so we're making their lives easier at the expense of the much smaller set of consumers that call set_boxed_logger directly. The docs that end up on docs.rs will include set_boxed_logger.