Add PL/Python functions for quoting strings

Add functions plpy.quote_ident, plpy.quote_literal,
plpy.quote_nullable, which wrap the equivalent SQL functions.

To be able to propagate char * constness properly, make the argument
of quote_literal_cstr() const char *.  This also makes it more
consistent with quote_identifier().

Jan Urbański, reviewed by Hitoshi Harada, some refinements by Peter
Eisentraut

src/pl/plpython/sql/plpython_quote.sql

@@ -0,0 +1,33 @@
+-- test quoting functions
+
+CREATE FUNCTION quote(t text, how text) RETURNS text AS $$
+    if how == "literal":
+        return plpy.quote_literal(t)
+    elif how == "nullable":
+        return plpy.quote_nullable(t)
+    elif how == "ident":
+        return plpy.quote_ident(t)
+    else:
+        raise plpy.Error("unrecognized quote type %s" % how)
+$$ LANGUAGE plpythonu;
+
+SELECT quote(t, 'literal') FROM (VALUES
+       ('abc'),
+       ('a''bc'),
+       ('''abc'''),
+       (''),
+       (''''),
+       ('xyzv')) AS v(t);
+
+SELECT quote(t, 'nullable') FROM (VALUES
+       ('abc'),
+       ('a''bc'),
+       ('''abc'''),
+       (''),
+       (''''),
+       (NULL)) AS v(t);
+
+SELECT quote(t, 'ident') FROM (VALUES
+       ('abc'),
+       ('a b c'),
+       ('a " ''abc''')) AS v(t);