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_test/src/ser.rs

@@ -15,12 +15,17 @@ use token::Token;
 #[derive(Debug)]
 pub struct Serializer<'a> {
     tokens: &'a [Token],
+    is_human_readable: bool,
 }
 
 impl<'a> Serializer<'a> {
     /// Creates the serializer.
     pub fn new(tokens: &'a [Token]) -> Self {
-        Serializer { tokens: tokens }
+        Serializer::readable(tokens, true)
+    }
+
+    pub fn readable(tokens: &'a [Token], is_human_readable: bool) -> Self {
+        Serializer { tokens: tokens, is_human_readable: is_human_readable }
     }
 
     /// Pulls the next token off of the serializer, ignoring it.
@@ -282,6 +287,10 @@ impl<'s, 'a> ser::Serializer for &'s mut Serializer<'a> {
             Ok(Variant { ser: self, end: Token::StructVariantEnd })
         }
     }
+
+    fn is_human_readable(&self) -> bool {
+        self.is_human_readable
+    }
 }
 
 pub struct Variant<'s, 'a: 's> {