to_char's ISO week

* to_char:

- full support for IW (ISO week) and vice versa conversion for IW too
(the to_char 'week' support is now complete and I hope correct).

Thomas, I use for IW code from timestamp.c, for this I create separate
function date2isoweek() from original 'case DTK_WEEK:' code in the
timestamp_part(). I mean will better use one code for same feature in
date_part() and in to_char(). The isoweek2date() is added to timestamp.c
too. Right?

IMHO in 7.1 will all to_char's features complete. It is cca 41 templates
for date/time and cca 21 for numbers.

* to_ascii:

- gcc, is it correct now? :-)

In the patch is documentation for to_char's IW and for to_ascii().

Karel