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/expected/plpython_test.out

@@ -43,9 +43,9 @@ contents.sort()
 return ", ".join(contents)
 $$ LANGUAGE plpythonu;
 select module_contents();
-                                      module_contents                                      
--------------------------------------------------------------------------------------------
- Error, Fatal, SPIError, debug, error, execute, fatal, info, log, notice, prepare, warning
+                                                            module_contents                                                            
+---------------------------------------------------------------------------------------------------------------------------------------
+ Error, Fatal, SPIError, debug, error, execute, fatal, info, log, notice, prepare, quote_ident, quote_literal, quote_nullable, warning
 (1 row)
 
 CREATE FUNCTION elog_test() RETURNS void