array_length(anyarray)

Hi,

Attached is a patch to add support for array_length(anyarray), which
only works for one-dimensional arrays, returns 0 for empty arrays and
complains if the array's lower bound isn't 1. In other words, does the
right thing when used with the arrays people use 99% of the time.

I'll add this to the next commit fest, but feel free to discuss it
before that.

Regards,
Marko Tiikkaja