plperl better array support

The attached patch (submitted for comment) is somewhat adapted from one
submitted last October. This allows returning a perl array where a
postgres array is expected.

example:

andrew=# create function blurfl() returns text[] language plperl as $$
andrew$# return ['a','b','c','a"b\c'];
andrew$# $$;
CREATE FUNCTION
andrew=# select blurfl();
blurfl
-------------------
{a,b,c,"a\"b\\c"}

Unlike the patch from October, this patch does not implement ANYARRAY or
ANYELEMENT pseudotypes. However it does escape/quote array elements
where necessary. It also preserves the old behaviour (if the plperl
function returns a string it is just passed through).

I'm not happy about constructing a string which we then parse out again
into an array - that strikes me as quite inefficient. (And there are
other inelegancies that I'd like to get rid of.) Much better would be to
use some array contruction calls directly - any pointers on how to do
that would be apprciated :-)

cheers

andrew