Make all objects Serializable

To avoid deadlocks when combining objects, lambdas and multi-threading,
lambdas in objects are compiled to instance methods of the module class
instead of static methods (see tests/run/deadlock.scala and
scala/scala-dev#195 for details).

This has worked well for us so far but this is problematic for
serialization: serializing a lambda requires serializing all the values
it captures, if this lambda is in an object, this means serializing the
enclosing object, which fails if the object does not extend
Serializable.

Because serializing objects is basically free since scala#5775, it seems like
the simplest solution is to simply make all objects Serializable, this
certainly seems preferable to deadlocks.

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -338,8 +338,9 @@ object desugar {
       case _ => false
     }
 
-    val isCaseClass  = mods.is(Case) && !mods.is(Module)
-    val isCaseObject = mods.is(Case) && mods.is(Module)
+    val isObject = mods.is(Module)
+    val isCaseClass  = mods.is(Case) && !isObject
+    val isCaseObject = mods.is(Case) && isObject
     val isImplicit = mods.is(Implicit)
     val isEnum = mods.isEnumClass && !mods.is(Module)
     def isEnumCase = mods.isEnumCase
@@ -522,6 +523,8 @@ object desugar {
       parents1 = enumClassTypeRef :: Nil
     if (isCaseClass | isCaseObject)
       parents1 = parents1 :+ scalaDot(str.Product.toTypeName) :+ scalaDot(nme.Serializable.toTypeName)
+    else if (isObject)
+      parents1 = parents1 :+ scalaDot(nme.Serializable.toTypeName)
     if (isEnum)
       parents1 = parents1 :+ ref(defn.EnumType)