[PATCH] psql: \dn+ to show size of each schema..

\db+ and \l+ show sizes of tablespaces and databases, so I was surprised in the
past that \dn+ didn't show sizes of schemas.  I would find that somewhat
convenient, and I assume other people would use it even more useful.

\db+ and \l+ seem to walk the filesystem, and this is distinguished from those
cases.  (Also, schemas are per-DB, not global).

Maybe it's an issue if \dn+ is slow and expensive, since that's how to display
ACL.  But \db+ has the same issue.  Maybe there should be a \db++ and \dn++ to
allow \dn+ to showing the ACL but not the size.

pg_relation_size() only includes one fork, and the other functions include
toast, which should be in its separate schema, so it has to be summed across
forks.

postgres=# \dnS+
 child              | postgres |                      |                                  | 946 MB
 information_schema | postgres | postgres=UC/postgres+|                                  | 88 kB
                    |          | =U/postgres          |                                  | 
 pg_catalog         | postgres | postgres=UC/postgres+| system catalog schema            | 42 MB
                    |          | =U/postgres          |                                  | 
 pg_toast           | postgres |                      | reserved schema for TOAST tables | 3908 MB
 public             | postgres | postgres=UC/postgres+| standard public schema           | 5627 MB
                    |          | =UC/postgres         |                                  |