Improve support for composite types in PL/Python.

Allow PL/Python functions to return arrays of composite types.
Also, fix the restriction that plpy.prepare/plpy.execute couldn't
handle query parameters or result columns of composite types.

In passing, adopt a saner arrangement for where to release the
tupledesc reference counts acquired via lookup_rowtype_tupdesc.
The callers of PLyObject_ToCompositeDatum were doing the lookups,
but then the releases happened somewhere down inside subroutines
of PLyObject_ToCompositeDatum, which is bizarre and bug-prone.
Instead release in the same function that acquires the refcount.

Ed Behn and Ronan Dunklau, reviewed by Abhijit Menon-Sen

src/pl/plpython/sql/plpython_composite.sql

@@ -125,7 +125,7 @@ SELECT * FROM changing_test();
 ALTER TABLE changing ADD COLUMN j integer;
 SELECT * FROM changing_test();
 
--- tables of composite types (not yet implemented)
+-- tables of composite types
 
 CREATE FUNCTION composite_types_table(OUT tab table_record[], OUT typ type_record[] ) RETURNS SETOF record AS $$
 yield {'tab': [['first', 1], ['second', 2]],