Bring back the Display requirement for Encodings

Basically, it's just really convenient and doing string formatting
without this requirement is a pain.

This reverts part of 2bc8f57

Author: SSheldon

README.md

@@ -34,17 +34,8 @@ assert!(parsed == &i32::encode());
 
 # Generating encoding strings
 
-The string representation of an `Encoding` can be generated via its `write`
-method:
-
-``` rust
-let mut result = String::new();
-i32::encode().write(&mut result).unwrap();
-assert_eq!(result, "i");
-```
-
-The encodings defined in this crate also implement `Display` for convenience,
-allowing the `to_string` method to be used:
+Every `Encoding` implements `Display` as its string representation.
+This can be generated conveniently through the `to_string` method:
 
 ``` rust
 assert_eq!(i32::encode().to_string(), "i");

src/encoding/never.rs

@@ -1,3 +1,5 @@
+use core::fmt;
+
 use {Descriptor, Encoding};
 use multi::{Encodings, EncodingsIterateCallback};
 
@@ -24,3 +26,9 @@ impl Encodings for Never {
         match self { }
     }
 }
+
+impl fmt::Display for Never  {
+    fn fmt(&self, _: &mut fmt::Formatter) -> fmt::Result {
+        match self { }
+    }
+}

src/lib.rs

@@ -37,18 +37,8 @@ assert!(parsed == &i32::encode());
 
 # Generating encoding strings
 
-The string representation of an `Encoding` can be generated via its `write`
-method:
-
-```
-# use objc_encode::{Encode, Encoding};
-let mut result = String::new();
-i32::encode().write(&mut result).unwrap();
-assert_eq!(result, "i");
-```
-
-The encodings defined in this crate also implement `Display` for convenience,
-allowing the `to_string` method to be used:
+Every `Encoding` implements `Display` as its string representation.
+This can be generated conveniently through the `to_string` method:
 
 ```
 # use objc_encode::Encode;
@@ -75,7 +65,7 @@ pub use descriptor::Descriptor;
 pub use multi::{Encodings, EncodingsIterateCallback};
 
 /// An Objective-C type encoding.
-pub trait Encoding {
+pub trait Encoding: fmt::Display {
     /// The type of `Encoding` that Self will use if it is an encoding for
     /// a pointer to describe its target.
     type PointerTarget: ?Sized + Encoding;