[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_ncp_buffer.cpp

@@ -30,8 +30,8 @@
 
 #include <openthread/openthread.h>
 
-#include "openthread-instance.h"
 #include "common/code_utils.hpp"
+#include "common/instance.hpp"
 #include "common/message.hpp"
 #include "ncp/ncp_buffer.hpp"
 
@@ -58,7 +58,7 @@ static const uint8_t sMottoText[]      = "Think good thoughts, say good words, d
 static const uint8_t sMysteryText[]    = "4871(\\):|(3$}{4|/4/2%14(\\)";
 static const uint8_t sHexText[]        = "0123456789abcdef";
 
-static otInstance *sInstance;
+static ot::Instance *sInstance;
 static MessagePool *sMessagePool;
 
 struct CallbackContext
@@ -378,7 +378,7 @@ void TestNcpFrameBuffer(void)
     NcpFrameBuffer::WritePosition pos1, pos2;
 
     sInstance = testInitInstance();
-    sMessagePool = &sInstance->mMessagePool;
+    sMessagePool = &sInstance->GetMessagePool();
 
     for (i = 0; i < sizeof(buffer); i++)
     {
@@ -1005,7 +1005,7 @@ void TestFuzzNcpFrameBuffer(void)
     uint32_t lensArrayCount[kNumPrios];
 
     sInstance = testInitInstance();
-    sMessagePool = &sInstance->mMessagePool;
+    sMessagePool = &sInstance->GetMessagePool();
 
     memset(buffer, 0, sizeof(buffer));