Cleanup the contrib/lo module: there is no need anymore to implement

a physically separate type.  Defining 'lo' as a domain over OID works
just fine and is more efficient.  Improve documentation and fix up the
test script.  (Would like to turn test script into a proper regression
test, but right now its output is not constant because of numeric OIDs;
plus it makes Unix-specific assumptions about files it can import.)