lib.data: make Field() immutable.

Mutability of Field isn't specified by the RFC and can cause issues
if the objects stored in Layout subclasses are mutated. There isn't
any reason to do that (the subclasses themselves are mutable and
handle that correctly), so disallow it.

tests/test_lib_data.py

@@ -59,6 +59,12 @@ class FieldTestCase(TestCase):
                 r"^Field offset must be a non-negative integer, not -1$"):
             Field(unsigned(2), -1)
 
+    def test_immutable(self):
+        with self.assertRaises(AttributeError):
+            Field(1, 0).shape = unsigned(2)
+        with self.assertRaises(AttributeError):
+            Field(1, 0).offset = 1
+
 
 class StructLayoutTestCase(TestCase):
     def test_construct(self):