PL/Python array support

Here is a patch to support arrays in PL/Python as parameters and return
values.  It converts an array parameter to a Python "list", and converts
a Python "sequence" return value back to an array.

I have settled on two implementation restrictions for the moment:

- Only supports one-dimensional arrays.  (Python has no multidimensional
lists, so the semantics of this would be dubious.)

- Does not support returning arrays of composite types.  (Basically too
complicated to implement right now and seemingly of limited practical
value.)