Class ActionFactory

object --+
         |
        ActionFactory

A factory class that will wrap up an arbitrary function
as an SCons-executable Action object.

The real heavy lifting here is done by the ActionCaller class.
We just collect the (positional and keyword) arguments that we're
called with and give them to the ActionCaller object we create,
so it can hang onto them until it needs them.

x.__init__(...) initializes x; see x.__class__.__doc__ for signature