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/bc.c

@@ -119,6 +119,11 @@ void mp_setup_code_state(mp_code_state_t *code_state, size_t n_args, size_t n_kw
     code_state->prev = NULL;
     #endif
 
+    #if MICROPY_PY_SYS_SETTRACE
+    code_state->prev_state = NULL;
+    code_state->frame = NULL;
+    #endif
+
     // get params
     size_t n_state = mp_decode_uint(&code_state->ip);
     code_state->ip = mp_decode_uint_skip(code_state->ip); // skip n_exc_stack