Introduce tower_grpc::metadata::MetadataMap type

As per the discussion in tower-rs#90, it wraps http::HeaderMap to abstract
away the fact that tower-grpc is HTTP based. At this time this class
is a pure wrapper that does not actually implement any of the gRPC
metadata specific stuff (such as binary fields), that will come later.

src/metadata_map.rs

@@ -220,7 +220,7 @@ impl MetadataMap {
     ///
     /// assert_eq!(2, map.len());
     ///
-    /// map.append("x-host-ip", "text/html".parse().unwrap());
+    /// map.append("x-mime-type", "text/html".parse().unwrap());
     ///
     /// assert_eq!(3, map.len());
     /// ```
@@ -241,12 +241,12 @@ impl MetadataMap {
     ///
     /// assert_eq!(0, map.keys_len());
     ///
-    /// map.insert("x-host-ip", "127.0.0.1".parse().unwrap());
-    /// map.insert("x-host-name", "localhost".parse().unwrap());
+    /// map.insert("x-mime", "text/plain".parse().unwrap());
+    /// map.insert("x-host", "localhost".parse().unwrap());
     ///
     /// assert_eq!(2, map.keys_len());
     ///
-    /// map.append("x-host-ip", "text/html".parse().unwrap());
+    /// map.insert("x-mime", "text/html".parse().unwrap());
     ///
     /// assert_eq!(2, map.keys_len());
     /// ```