Implement display for Token and PartialToken

Relates to #19

src/token/display.rs

@@ -0,0 +1,57 @@
+use std::fmt;
+use token::{PartialToken, Token};
+
+impl fmt::Display for Token {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        use self::Token::*;
+        match self {
+            Plus => write!(f, "+"),
+            Minus => write!(f, "-"),
+            Star => write!(f, "*"),
+            Slash => write!(f, "/"),
+            Percent => write!(f, "%"),
+            Hat => write!(f, "^"),
+
+            // Logic
+            Eq => write!(f, "=="),
+            Neq => write!(f, "!="),
+            Gt => write!(f, ">"),
+            Lt => write!(f, "<"),
+            Geq => write!(f, ">="),
+            Leq => write!(f, "<="),
+            And => write!(f, "&&"),
+            Or => write!(f, "||"),
+            Not => write!(f, "!"),
+
+            // Precedence
+            LBrace => write!(f, "("),
+            RBrace => write!(f, ")"),
+
+            // Aggregation
+            Comma => write!(f, ","),
+
+            // Values => write!(f, ""), Variables and Functions
+            Identifier(identifier) => identifier.fmt(f),
+            Float(float) => float.fmt(f),
+            Int(int) => int.fmt(f),
+            Boolean(boolean) => boolean.fmt(f),
+        }
+    }
+}
+
+impl fmt::Display for PartialToken {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        use self::PartialToken::*;
+        match self {
+            Token(token) => token.fmt(f),
+            Literal(literal) => literal.fmt(f),
+            Whitespace => write!(f, " "),
+            Eq => write!(f, "="),
+            ExclamationMark => write!(f, "!"),
+            Gt => write!(f, ">"),
+            Lt => write!(f, "<"),
+            Ampersand => write!(f, "&"),
+            VerticalBar => write!(f, "|"),
+        }
+    }
+}

src/token/mod.rs

@@ -1,6 +1,8 @@
 use error::Error;
 use value::{FloatType, IntType};
 
+mod display;
+
 #[derive(Clone, PartialEq, Debug)]
 pub enum Token {
     // Arithmetic