Refactor SkylarkList to allow MutableList

Make SkylarkList no longer read-only to match Python and the BUILD language.
Instead, subject it to a Mutability object inherited from the Environment.

--
MOS_MIGRATED_REVID=103332973
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/Runtime.java b/src/main/java/com/google/devtools/build/lib/syntax/Runtime.java
index 098836a..ca83d65 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/Runtime.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/Runtime.java
@@ -42,6 +42,8 @@
   /**
    * There should be only one instance of this type to allow "== None" tests.
    */
+  @SkylarkModule(name = "NoneType", documented = false,
+    doc = "Unit type, containing the unique value None")
   @Immutable
   public static final class NoneType implements SkylarkValue {
     private NoneType() {}