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/client/mod.rs

@@ -793,13 +793,11 @@ fn absolute_form(uri: &mut Uri) {
 }
 
 fn authority_form(uri: &mut Uri) {
-    if log_enabled!(::log::Level::Warn) {
-        if let Some(path) = uri.path_and_query() {
-            // `https://hyper.rs` would parse with `/` path, don't
-            // annoy people about that...
-            if path != "/" {
-                warn!("HTTP/1.1 CONNECT request stripping path: {:?}", path);
-            }
+    if let Some(path) = uri.path_and_query() {
+        // `https://hyper.rs` would parse with `/` path, don't
+        // annoy people about that...
+        if path != "/" {
+            warn!("HTTP/1.1 CONNECT request stripping path: {:?}", path);
         }
     }
     *uri = match uri.authority() {