Convert Postgres arrays to Perl arrays on PL/perl input arguments

More generally, arrays are turned in Perl array references, and row and
composite types are turned into Perl hash references.  This is done
recursively, in a way that's natural to every Perl programmer.

To avoid a backwards compatibility hit, the string representation of
each structure is also available if the function requests it.

Authors: Alexey Klyukin and Alex Hunsaker.
Some code cleanups by me.

src/pl/plperl/plperl.h

@@ -59,6 +59,7 @@ HV		   *plperl_spi_exec_prepared(char *, HV *, int, SV **);
 SV		   *plperl_spi_query_prepared(char *, int, SV **);
 void		plperl_spi_freeplan(char *);
 void		plperl_spi_cursor_close(char *);
+char 	   *plperl_sv_to_literal(SV *, char *);