Expand varargs when binding function arguments

[epatters]

When recording a flow graph, *args and **kwargs arguments should be "expanded" as part of binding a function call to a function signature.

For example, one function that is currently broken is numpy's meshgrid, which has signature:

np.meshgrid(*xi, **kwargs)

Currently, the call np.meshgrid(x, y, copy=True) is bound as

xi=(x, y), kwargs={'copy': True}

Under our programming model, the call should be bound as something like

xi__0=x, xi__1=y, copy=True