hokey primitive to create temporary file

hokey primitive to create temporary file

The trouble with os.tmpname() is that it always creates in /tmp.
If /tmp is in a different volume from our real filename, os.rename()
will fail.

This new primitive doesn't support primitive paths yet.

I'm also again nervous about the security implications of my whole
approach. What if we create an inner function called start_writing?
Would we be able to do anything inside it? I'm starting to suspect this
whole approach of going by caller name is broken. An app could also
create inner functions called 'main'..