Support multi-dimensional arrays in PL/python.

Multi-dimensional arrays can now be used as arguments to a PL/python function
(used to throw an error), and they can be returned as nested Python lists.

This makes a backwards-incompatible change to the handling of composite
types in arrays. Previously, you could return an array of composite types
as "[[col1, col2], [col1, col2]]", but now that is interpreted as a two-
dimensional array. Composite types in arrays must now be returned as
Python tuples, not lists, to resolve the ambiguity. I.e. "[(col1, col2),
(col1, col2)]".

To avoid breaking backwards-compatibility, when not necessary, () is still
accepted for arrays at the top-level, but it is always treated as a
single-dimensional array. Likewise, [] is still accepted for composite types,
when they are not in an array. Update the documentation to recommend using []
for arrays, and () for composite types, with a mention that those other things
are also accepted in some contexts.

This needs to be mentioned in the release notes.

Alexey Grishchenko, Dave Cramer and me. Reviewed by Pavel Stehule.

Discussion: <CAH38_tmbqwaUyKs9yagyRra=SMaT45FPBxk1pmTYcM0TyXGG7Q@mail.gmail.com>

src/pl/plpython/sql/plpython_composite.sql

@@ -169,13 +169,13 @@ SELECT * FROM changing_test();
 -- tables of composite types
 
 CREATE FUNCTION composite_types_table(OUT tab table_record[], OUT typ type_record[] ) RETURNS SETOF record AS $$
-yield {'tab': [['first', 1], ['second', 2]],
+yield {'tab': [('first', 1), ('second', 2)],
       'typ': [{'first': 'third', 'second': 3},
               {'first': 'fourth', 'second': 4}]}
-yield {'tab': [['first', 1], ['second', 2]],
+yield {'tab': [('first', 1), ('second', 2)],
       'typ': [{'first': 'third', 'second': 3},
               {'first': 'fourth', 'second': 4}]}
-yield {'tab': [['first', 1], ['second', 2]],
+yield {'tab': [('first', 1), ('second', 2)],
       'typ': [{'first': 'third', 'second': 3},
               {'first': 'fourth', 'second': 4}]}
 $$ LANGUAGE plpythonu;
@@ -207,3 +207,9 @@ SELECT * FROM return_record_2('v4') AS (v1 int, v3 int, v2 int);
 -- works
 SELECT * FROM return_record_2('v3') AS (v1 int, v3 int, v2 int);
 SELECT * FROM return_record_2('v3') AS (v1 int, v2 int, v3 int);
+
+-- multi-dimensional array of composite types.
+CREATE FUNCTION composite_type_as_list()  RETURNS type_record[] AS $$
+  return [[('first', 1), ('second', 1)], [('first', 2), ('second', 2)], [('first', 3), ('second', 3)]];
+$$ LANGUAGE plpythonu;
+SELECT * FROM composite_type_as_list();