Fix for SerialVersionUID instability.

Can hardly believe this has been broken for a decade or so, but
there it is - see test case. Four classes attempt to set their
SerialVersionUID to 13. One succeeds. No warnings or errors. The
output before this patch (for me anyway - your random numbers may
differ) was:

  860336111422349646
  13
  8409527228024057943
  -7852527872932878365

There was already code in place for rejecting annotations
with non-constant args when constant args are required, but
that check is only performed on ClassfileAnnotations, and
SerialVersionUID was a StaticAnnotation. Maybe people don't
reach for ClassfileAnnotation because of this giant warning
which I see no way to suppress:

  warning: Implementation restriction: subclassing Classfile does
  not make your annotation visible at runtime. If that is what you
  want, you must write the annotation class in Java.

Why did I change the name of the field from uid to value?
If you don't use the name 'value', you have to name the argument
every time you use it, even if it's the only parameter. I didn't
relish breaking every usage of scala's @serialversionuid in the
known universe.

Author: paulp

src/library/scala/SerialVersionUID.scala

@@ -12,4 +12,4 @@ package scala
  * Annotation for specifying the `static SerialVersionUID` field
  * of a serializable class.
  */
-class SerialVersionUID(uid: Long) extends scala.annotation.StaticAnnotation
+class SerialVersionUID(value: Long) extends scala.annotation.ClassfileAnnotation

test/files/neg/serialversionuid-not-const.check

@@ -0,0 +1,10 @@
+serialversionuid-not-const.scala:1: error: annotation argument needs to be a constant; found: 13L.toLong
+@SerialVersionUID(13l.toLong) class C1 extends Serializable
+                      ^
+serialversionuid-not-const.scala:3: error: annotation argument needs to be a constant; found: 13.asInstanceOf[Long]
+@SerialVersionUID(13.asInstanceOf[Long]) class C3 extends Serializable
+                                 ^
+serialversionuid-not-const.scala:4: error: annotation argument needs to be a constant; found: Test.bippy
+@SerialVersionUID(Test.bippy) class C4 extends Serializable
+                       ^
+three errors found

test/files/neg/serialversionuid-not-const.scala

@@ -0,0 +1,16 @@
+@SerialVersionUID(13l.toLong) class C1 extends Serializable
+@SerialVersionUID(13l) class C2 extends Serializable
+@SerialVersionUID(13.asInstanceOf[Long]) class C3 extends Serializable
+@SerialVersionUID(Test.bippy) class C4 extends Serializable
+
+object Test {
+  val bippy = 13L
+
+  def show(c: Class[_]) = println(java.io.ObjectStreamClass.lookup(c).getSerialVersionUID)
+  def main(args: Array[String]): Unit = {
+    show(classOf[C1])
+    show(classOf[C2])
+    show(classOf[C3])
+    show(classOf[C4])
+  }
+}