Further plperl cleanup: be more paranoid about checking the type of

data returned from Perl.  Consolidate multiple bits of code to convert
a Perl hash to a tuple, and drive the conversion off the keys present
in the hash rather than the tuple column names, so we detect error if
the hash contains keys it shouldn't.  (This means keys not in the hash
will silently default to NULL, which seems ok to me.)  Fix a bunch of
reference-count leaks too.