## Motivation
As discussed in #383 , adds the new `set_default` method.
## Solution
* Add `tracing::subscriber::set_default` which sets the default
subscriber and returns a drop guard. This drop guard will reset the
dispatch on drop.
* Add `tracing_core::dispatcher::set_default` method which sets the
default dispatch and returns a drop guard.
* Update `tracing_core::dispatcher::with_default` method to use the new
`tracing_core::dispatcher::set_default` method.
* Add test to confirm expected behavior
Fixes: #383
