Avoid class initialization traps in the object wrapper by
extending App.

Previously, the following line would hang in the REPL
because the parallel operation was started from the constructor
of the wrapping object, i.e., from the static initializer of
the class. Class-loading the closure would deadlock on loading
the wrapper, which in turn blocks on completion of the par.map.

```
scala> def i = 42 ; List(1, 2, 3).par.map(x => x + i)
```

Any user code that starts a thread could deadlock, including
innocent experiments with Futures in for comprehensions.

The App object is initialized in the usual place, namely
the print method of the eval object, by invoking main.
(A lazy `compute` accommodates two paths of evaluation:
`print` for REPL, `result` for script engine.)

A compiler option `-YreplWrap` takes values "class", "object"
and "app" (default) to select the wrapper template.

```
scala> 42 // show
object $read extends scala.AnyRef {
  def <init>() = {
    super.<init>;
    ()
  };
  object $iw extends App {
    def <init>() = {
      super.<init>;
      ()
    };
    val res0 = 42
  };
}
[[syntax trees at end of                     typer]] // <console>
package $line3 {
  object $read extends scala.AnyRef {
    def <init>(): $line3.$read.type = {
      $read.super.<init>();
      ()
    };
    object $iw extends AnyRef with App {
      def <init>(): type = {
        $iw.super.<init>();
        ()
      };
      private[this] val res0: Int = 42;
      <stable> <accessor> def res0: Int = $iw.this.res0
    }
  }
}

[[syntax trees at end of                     typer]] // <console>
package $line3 {
  object $eval extends scala.AnyRef {
    def <init>(): $line3.$eval.type = {
      $eval.super.<init>();
      ()
    };
    <stable> <accessor> lazy def compute: Unit = $line3.$read.$iw.main(null);
    lazy private[this] var $result: Int = _;
    <stable> <accessor> lazy def $result: Int = {
      $eval.this.$result = {
        $eval.this.compute;
        $line3.$read.$iw.res0
      };
      $eval.this.$result
    };
    lazy private[this] var $print: String = _;
    <stable> <accessor> lazy def $print: String = {
      $eval.this.$print = {
        $eval.this.compute;
        "res0: Int = ".+(scala.runtime.ScalaRunTime.replStringOf($line3.$read.$iw.res0, 1000))
      };
      $eval.this.$print
    }
  }
}

res0: Int = 42

```