Add support for storing args during an exception raised by an irq.

The user code should call micropython.alloc_emergency_exception_buf(size)
where size is the size of the buffer used to print the argument
passed to the exception.

With the test code from #732, and a call to
micropython.alloc_emergenncy_exception_buf(100) the following error is
now printed:
```python
>>> import heartbeat_irq
Uncaught exception in Timer(4) interrupt handler
Traceback (most recent call last):
  File "0://heartbeat_irq.py", line 14, in heartbeat_cb
NameError: name 'led' is not defined
```

1 files changed, 2 insertions, 0 deletions

diff --git a/py/obj.h b/py/obj.h
index d0284e6d21..5f347a2674 100644
--- a/py/obj.h
+++ b/py/obj.h
@@ -453,6 +453,8 @@ void mp_obj_exception_add_traceback(mp_obj_t self_in, qstr file, mp_uint_t line,
 void mp_obj_exception_get_traceback(mp_obj_t self_in, mp_uint_t *n, mp_uint_t **values);
 mp_obj_t mp_obj_exception_get_value(mp_obj_t self_in);
 mp_obj_t mp_obj_exception_make_new(mp_obj_t type_in, uint n_args, uint n_kw, const mp_obj_t *args);
+mp_obj_t mp_alloc_emergency_exception_buf(mp_obj_t size_in);
+void mp_init_emergency_exception_buf(void);
 
 // str
 mp_obj_t mp_obj_str_builder_start(const mp_obj_type_t *type, uint len, byte **data);