asyncio: Tulip issue 173: Enhance repr(Handle) and repr(Task)

repr(Handle) is shorter for function: "foo" instead of "<function foo at
0x...>". It now also includes the source of the callback, filename and line
number where it was defined, if available.

repr(Task) now also includes the current position in the code, filename and
line number, if available. If the coroutine (generator) is done, the line
number is omitted and "done" is added.