[locator] adding Get<Type> to InstanceLocator (#3714)

This commit changes how the objects in OpenThread access each other.
It adds a template `Get<Type>()` method in `InstanceLocator`. This
method returns a reference to a given `Type` object belonging to the
OpenThread instance (e.g. `Get<MeshForwarder>()` returns a reference
to `MeshForwarder` object on the OpenThread instance). The
`InstanceLocator` is used as base class of all OpenThread classes so
every class can easily access any other object. This commit also
changes how the main instance is retrieved in `InstanceLocator` for
the single-instance case. The method `GetInstance()` directly uses the
raw buffer `gInstanceRaw`. This change helps make the `GetInstance()`
and in turn all `Get<Type>()` methods in-line. This commit also
removes the existing getters across all classes to use the new
`Get<Type>()` model.

tests/unit/test_priority_queue.cpp

@@ -220,7 +220,7 @@ void TestPriorityQueue(void)
     instance = testInitInstance();
     VerifyOrQuit(instance != NULL, "Null OpenThread instance\n");
 
-    messagePool = &instance->GetMessagePool();
+    messagePool = &instance->Get<ot::MessagePool>();
 
     // Use the function "New()" to allocate messages with different priorities
     for (int i = 0; i < kNumNewPriorityTestMessages; i++)