Bug in ALTER COLUMN/TYPE

I think we need to deny changing column types if a function is using the 
table type as a return set.

test=# create table test (a int4);
CREATE TABLE
test=# create function test () returns setof test as 'select 1' language 
sql;
CREATE FUNCTION
test=# alter table test alter a type bigint;
ALTER TABLE
test=# select * from test();
ERROR:  return type mismatch in function declared to return test
DETAIL:  Final SELECT returns integer instead of bigint at column 1.
CONTEXT:  SQL function "test" during startup

Chris