Serialize to binary if the serde format is not human readable

This implements the KISS suggested in https://github.com/serde-rs/serde/issues/790.
It is possible that one of the other approaches may be better but this
seemed like the simplest one to reignite som discussion.

Personally I find the original suggestion of adding two traits perhaps slightly
cleaner in theory but I think it ends up more complicated in the end
since the added traits also need to be duplicated to to the `Seed`
traits.

Closes #790

serde/src/de/mod.rs

@@ -1011,6 +1011,12 @@ pub trait Deserializer<'de>: Sized {
     fn deserialize_ignored_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
     where
         V: Visitor<'de>;
+
+    /// Returns whether the serialized data is human readable or not.
+    ///
+    /// Some formats are not intended to be human readable. For these formats
+    /// a type being serialized may opt to serialize into a more compact form.
+    fn is_human_readable(&self) -> bool { true }
 }
 
 ////////////////////////////////////////////////////////////////////////////////

serde/src/ser/mod.rs

@@ -1363,6 +1363,12 @@ pub trait Serializer: Sized {
     fn collect_str<T: ?Sized>(self, value: &T) -> Result<Self::Ok, Self::Error>
     where
         T: Display;
+
+    /// Returns wheter the data format is human readable or not.
+    ///
+    /// Some formats are not intended to be human readable. For these formats
+    /// a type being serialized may opt to serialize into a more compact form.
+    fn is_human_readable(&self) -> bool { true }
 }
 
 /// Returned from `Serializer::serialize_seq`.