py/nlr: Implement jump callbacks.

NLR buffers are usually quite large (use lots of C stack) and expensive to
push and pop.  Some of the time they are only needed to perform clean up if
an exception happens, and then they re-raise the exception.

This commit allows optimizing that scenario by introducing a linked-list of
NLR callbacks that are called automatically when an exception is raised.
They are essentially a light-weight NLR handler that can implement a
"finally" block, i.e. clean-up when an exception is raised, or (by passing
`true` to nlr_pop_jump_callback) when execution leaves the scope.

Signed-off-by: Damien George <damien@micropython.org>

py/mpstate.h

@@ -271,6 +271,7 @@ typedef struct _mp_state_thread_t {
     mp_obj_dict_t *dict_globals;
 
     nlr_buf_t *nlr_top;
+    nlr_jump_callback_node_t *nlr_jump_callback_top;
 
     // pending exception object (MP_OBJ_NULL if not pending)
     volatile mp_obj_t mp_pending_exception;