Parse string concatenation

Parser.cs

@@ -142,7 +142,9 @@ class Parser
 
 	private Expr sum() => binaryLeft(product, TokenType.Minus, TokenType.Plus);
 
-	private Expr product() => binaryLeft(unary, TokenType.Slash, TokenType.Asterisk);
+	private Expr product() => binaryLeft(concat, TokenType.Slash, TokenType.Asterisk);
+
+	private Expr concat() => binaryLeft(unary, TokenType.PlusPlus);
 
 	private Expr unary()
 	{