[IronPython] Improvements to the hosting APIs

[Jeff Hardy]

Hi all,
Dave F. mentioned on Twitter that he was looking at the hosting APIs,
and the made the mistake of accepting my unsolicited advice. So, here
are a couple of my wishes (both form NWSGI):

* A version of PythonOps.MakeException that doesn't take a code
context - It's not hard to create one, but every time I do I feel like
I'm doing something wrong.
* An easy way to wrap a Stream as a PythonFile.

I don't know where these make the most sense, but engine.MakeException
and engine.MakeFile both seem right to me.

Other than that, the hosting APIs are very nice to use. I'm really
reaching to come up with those issues.

- Jeff