[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.

src/core/api/logging_api.cpp

@@ -37,6 +37,7 @@
 
 #include <openthread/logging.h>
 #include "common/instance.hpp"
+#include "common/locator-getters.hpp"
 
 using namespace ot;