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/mpstate.h

@@ -253,6 +253,12 @@ typedef struct _mp_state_thread_t {
     mp_obj_dict_t *dict_globals;
 
     nlr_buf_t *nlr_top;
+
+    #if MICROPY_PY_SYS_SETTRACE
+    mp_obj_t prof_trace_callback;
+    bool prof_callback_is_executing;
+    struct _mp_code_state_t *current_code_state;
+    #endif
 } mp_state_thread_t;
 
 // This structure combines the above 3 structures.