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/test/java/com/google/devtools/build/lib/syntax/SkylarkEvaluationTest.java b/src/test/java/com/google/devtools/build/lib/syntax/SkylarkEvaluationTest.java
index 2ebd7df..1cc4812 100644
--- a/src/test/java/com/google/devtools/build/lib/syntax/SkylarkEvaluationTest.java
+++ b/src/test/java/com/google/devtools/build/lib/syntax/SkylarkEvaluationTest.java
@@ -915,7 +915,8 @@
 
   @Test
   public void testListAnTupleConcatenationDoesNotWorkInSkylark() throws Exception {
-    new SkylarkTest().testIfExactError("cannot concatenate lists and tuples", "[1, 2] + (3, 4)");
+    new SkylarkTest().testIfExactError("can only concatenate list (not \"tuple\") to list",
+        "[1, 2] + (3, 4)");
   }
 
   @Test