New debugger features!

Python now allows chained exceptions, both from explicit "from"
chaining, and implicit exceptions from within exception handlers.

Now you can switch to the chained exceptions traceback to inspect it.
A command "subshell" is used for this.
Also enhanced some reporting commands. The whatis command shows more
information about an object.

Make better use of the new features of the inspect module.

TODO: better coroutine debugging.