Here's an 0.4 version, in which I've written some user docs, refactored
the array-specific code into a more reasonable arrangement, and adjusted
a lot of the built-in array functions to support expanded arrays directly.
This is about as far as I feel a need to take the latter activity, at
least for now; there are a few remaining operations that might be worth
converting but it's not clear they'd really offer much benefit.
I think this is actually now a serious candidate to be committed as-is,
not just a prototype. What we lack though is a clear understanding of
the performance characteristics.
regards, tom lane