Add some missing common functions

[sgrif]

We have max. We don't have min. Or avg, sum, etc. We should add these. They should not use the sql_function! macro, as they need to accept more than one argument type. min should have the same bounds as max. Others might need new bounds. I'm fine with just naming the trait something like CanBePassedToAvg or something, if there's not a good generic name. If you ever need to find the types which can be passed to a function in postgres, just open up psql and run this query:

select typname from pg_type where oid in (select proargtypes::text::oid from pg_proc where proname = 'sum');