temporal version of generate_series()

I found a TODO item "Add temporal versions of generate_series()" and
wrote patch for generate_series(timestamp, timestamp, interval).

I just copied it from int4.c to timestamp.c and fit the data types in
timestamp and interval. I wonder if we need ones for timestamptz and
time?? If so, the implementation will be so similar that some kind of
generic approach should make things smart. For example, find add '+'
operator function from syscache and take anyelement as arguments.

# This is my first time to send a patch. If I did something wrong, I
appreciate your pointing me out.

Hitoshi Harada