py/objexcept: Support errno attribute on OSError exceptions.

This commit adds the errno attribute to exceptions, so code can retrieve
errno codes from an OSError using exc.errno.

The implementation here simply lets `errno` (and the existing `value`)
attributes work on any exception instance (they both alias args[0]).  This
is for efficiency and to keep code size down.  The pros and cons of this
are:

Pros:
- more compatible with CPython, less difference to document and learn
- OSError().errno will correctly return None, whereas the current way of
  doing it via OSError().args[0] will raise an IndexError
- it reduces code size on most bare-metal ports (because they already have
  the errno qstr)
- for Python code that uses exc.errno the generated bytecode is 2 bytes
  smaller and more efficient to execute (compared with exc.args[0]); so
  bytecode loaded to RAM saves 2 bytes RAM for each use of this attribute,
  and bytecode that is frozen saves 2 bytes flash/ROM for each use
- it's easier/shorter to type, and saves 2 bytes of space in .py files that
  use it (for each use)

Cons:
- increases code size by 4-8 bytes on minimal ports that don't already have
  the `errno` qstr
- all exceptions now have .errno and .value attributes (a cpydiff test is
  added to address this)

See also #2407.

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

tests/basics/subclass_native3.py

@@ -1,6 +1,10 @@
+# test subclassing a native exception
+
+
 class MyExc(Exception):
     pass
 
+
 e = MyExc(100, "Some error")
 print(e)
 print(repr(e))
@@ -20,3 +24,19 @@ try:
     raise MyExc("Some error2")
 except:
     print("Caught user exception")
+
+
+class MyStopIteration(StopIteration):
+    pass
+
+
+print(MyStopIteration().value)
+print(MyStopIteration(1).value)
+
+
+class MyOSError(OSError):
+    pass
+
+
+print(MyOSError().errno)
+print(MyOSError(1, "msg").errno)