Private classes, enum `wireNumber`, memoizable `hashCode`

• Support private Built classes. Note that private classes cannot be made
  serializable.
• Support serializing enums to ints: add wireNumber to
  @BuiltValueEnumConst.
• Support memoizing hashCode, so it's computed lazily once. Write an abstract
  getter int get hashCode; then annotate it with @memoized to turn this on
  for a built_value class.
• Trim built_value_test dependencies: depend on matcher instead of test.
• Fix enum generator error messages when value and valueOf are missing.