The great un-unwrappening

[boustrophedon]

Since 99% of the time you're just going to call unwrap or expect on the result of enable/apply calls, for the next release I'd like to rename enable to try_enable, etc., and then make enable unwrap the result internally.

The demo in the readme would just look like

    extrasafe::SafetyContext::new()
        .enable(
           extrasafe::builtins::SystemIO::nothing()
              .allow_stdout()
              .allow_stderr()
   	    )
	.apply_to_all_threads();