py: Integrate sys.settrace feature into the VM and runtime.

This commit adds support for sys.settrace, allowing to install Python
handlers to trace execution of Python code.  The interface follows CPython
as closely as possible.  The feature is disabled by default and can be
enabled via MICROPY_PY_SYS_SETTRACE.

py/runtime.c

@@ -125,6 +125,12 @@ void mp_init(void) {
     MP_STATE_VM(sys_exitfunc) = mp_const_none;
     #endif
 
+    #if MICROPY_PY_SYS_SETTRACE
+    MP_STATE_THREAD(prof_trace_callback) = MP_OBJ_NULL;
+    MP_STATE_THREAD(prof_callback_is_executing) = false;
+    MP_STATE_THREAD(current_code_state) = NULL;
+    #endif
+
     #if MICROPY_PY_THREAD_GIL
     mp_thread_mutex_init(&MP_STATE_VM(gil_mutex));
     #endif