Add echo method to printers.

It's often useful to print a value as it is returned. This method
avoids the temporary variable to hold the vaue before it is printed
and then return. I.e.

    printer.echo(msg, expr)

instead of

    val x = expr
    printer.println(msg + x)
    x

Author: odersky

src/dotty/tools/dotc/config/Printers.scala

@@ -4,10 +4,12 @@ object Printers {
 
   class Printer {
     def println(msg: => String): Unit = System.out.println(msg)
+    def echo[T](msg: => String, value: T): T = { println(msg + value); value }
   }
 
   object noPrinter extends Printer {
     override def println(msg: => String): Unit = ()
+    override def echo[T](msg: => String, value: T): T = value
   }
 
   val default: Printer = new Printer