Make array.zeros work for custom types

@mattwala How many computer scientists does it take to zero-initialize a buffer? Hrmph. Turns out the argument to clEnqueueFillBuffer has to be power-of-two sized, so it doesn't exactly solve the general case. But this should do for the moment.

The other depressing fact I realized is that even fixing the code generated for fill isn't sufficient--that then runs into the LLVM issue with struct passing.

Thanks for getting this working.