Provide script engine rendering object with parent class loader

The point of this change is to make it possible to cast script engine's output
to classes from the calling context, which is currently impossible due to lack
of a common parent classloader, see:

https://stackoverflow.com/questions/2591779/cast-across-classloader

Author: rjolly

compiler/src/dotty/tools/repl/ScriptEngine.scala

@@ -24,7 +24,7 @@ class ScriptEngine extends AbstractScriptEngine {
       "-color:never",
       "-Xrepl-disable-display"
     ), Console.out, None)
-  private val rendering = new Rendering
+  private val rendering = new Rendering(Some(getClass.getClassLoader))
   private var state: State = driver.initialState
 
   def getFactory: ScriptEngineFactory = new ScriptEngine.Factory

tests/run-with-compiler/scripting.check

@@ -1 +1,2 @@
 42
+42

tests/run-with-compiler/scripting.scala

@@ -3,5 +3,6 @@ object Test {
     val m = new javax.script.ScriptEngineManager(getClass().getClassLoader())
     val e = m.getEngineByName("scala")
     println(e.eval("42"))
+    println(e.eval("Some(42)").asInstanceOf[Option[Int]].get)
   }
 }