nit(app): consolidate impl Config blocks (#3919)

this is a trivial, cosmetic change.

`Config` has two consecutive `impl` blocks in the `linkerd-app` library.
these do not include distinct generics or trait bounds, so the methods
contained therein do not need to live in two distinct `impl` blocks.

this commit consolidates these blocks.

while we are performing this change, we add two `=== impl T ===`
banners, which are used throughout the project as greppable strings to
find methods and trait implementations for a given type.

Signed-off-by: katelyn martin <kate@buoyant.io>

Author: cratelyn

linkerd/app/src/lib.rs

@@ -83,13 +83,13 @@ pub struct App {
     tap: tap::Tap,
 }
 
+// === impl Config ===
+
 impl Config {
     pub fn try_from_env() -> Result<Self, env::EnvError> {
         env::Env.try_config()
     }
-}
 
-impl Config {
     /// Build an application.
     ///
     /// It is currently required that this be run on a Tokio runtime, since some
@@ -358,6 +358,8 @@ impl Config {
     }
 }
 
+// === impl App ===
+
 impl App {
     pub fn admin_addr(&self) -> Local<ServerAddr> {
         self.admin.listen_addr