Add ObjBrace token

To distinguish between objects and code bodies, an object is delimited
by curly braces, prefixed with a percent sign. Ex:

    obj = %{
        key = value
    }

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/syn/token.rs

@@ -16,6 +16,7 @@ pub enum TokenKind {
     RBrace,
     LBracket,
     RBracket,
+    ObjBrace,
     Comma,
 
     Eq,
@@ -50,6 +51,7 @@ impl Display for TokenKind {
             RParen => "right paren",
             LBrace => "left brace",
             RBrace => "right brace",
+            ObjBrace => "object brace",
             LBracket => "left bracket",
             RBracket => "right bracket",
             Comma => "comma",