[instance] define ot::Instance class (#2307)

This commit makes the following changes: It defines the public
`otInstance` as an empty opaque structure which is used by all public
C OpenThread APIs. It defines a new class `ot::Instance` (inheriting
from `otInstance) which is then used in core source files. The
functionality related to the instance is also moved/added into the
newly added `Instance` class (as class/static or member methods).

tests/unit/test_spinel_encoder.cpp

@@ -30,8 +30,8 @@
 
 #include <openthread/openthread.h>
 
-#include "openthread-instance.h"
 #include "common/code_utils.hpp"
+#include "common/instance.hpp"
 #include "ncp/spinel_encoder.hpp"
 
 #include "test_util.h"