[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_platform.cpp

@@ -77,7 +77,7 @@ void testPlatResetToDefaults(void)
     g_testPlatRadioGetTransmitBuffer = NULL;
 }
 
-otInstance *testInitInstance(void)
+ot::Instance *testInitInstance(void)
 {
     otInstance *instance = NULL;
 
@@ -99,7 +99,7 @@ otInstance *testInitInstance(void)
     instance = otInstanceInitSingle();
 #endif
 
-    return instance;
+    return static_cast<ot::Instance *>(instance);
 }
 
 void testFreeInstance(otInstance *aInstance)