feat(lib): Move from log to tracing in a backwards-compatible way (#2204)

I've moved Hyper from `log` to `tracing`. Existing `log`-based users shouldn't notice a difference, but `tracing` users will see higher performance when filtering data. This isn't the _end_  of the `tracing` integration that can happen in `Hyper` (e.g., Hyper can start using spans, typed fields, etc.), but _something_ is better than nothing. I'd rather address those points, including examples, in followups.

I've attached a screenshot of the `hello` example working, but the logged information is pulled from `tracing`, not `log`.

<img width="514" alt="Screen Shot 2020-05-16 at 1 23 19 PM" src="https://user-images.githubusercontent.com/2067774/82126298-d8103800-9779-11ea-8f0b-57c632c684d6.png">

src/lib.rs

@@ -43,7 +43,7 @@
 #[doc(hidden)]
 pub use http;
 #[macro_use]
-extern crate log;
+extern crate tracing;
 
 #[cfg(all(test, feature = "nightly"))]
 extern crate test;