Allow dropping multiple functions at once

The generic drop support already supported dropping multiple objects of
the same kind at once.  But the previous representation
of function signatures across two grammar symbols and structure members
made this cumbersome to do for functions, so it was not supported.  Now
that function signatures are represented by a single structure, it's
trivial to add this support.  Same for aggregates and operators.

Reviewed-by: Jim Nasby <Jim.Nasby@BlueTreble.com>
Reviewed-by: Michael Paquier <michael.paquier@gmail.com>

src/test/regress/sql/create_function_3.sql

@@ -156,6 +156,8 @@ SELECT routine_name, ordinal_position, parameter_name, parameter_default
     WHERE routine_schema = 'temp_func_test' AND routine_name ~ '^functest_is_'
     ORDER BY 1, 2;
 
+DROP FUNCTION functest_IS_1(int, int, text), functest_IS_2(int), functest_IS_3(int);
+
 
 -- Cleanups
 DROP SCHEMA temp_func_test CASCADE;