Dump serials as serial -- not a sequence

- Fix an issue indexing domains. Ensure we look for the operator class
of the base type, not the domain type.
- Add an is_serial boolean attribute to sequence tables.
- Add serial4 and serial8 domains with respective serial and bigserial
aliases (similar to int4 and int8 to integer and bigint)
- Jump through hoops to deal with a primary key'd serial column, alter
table add primary key doesn't work for these.
- Fix type sanity checks to account for domains.

End result, pg_dump no longer uses sequences for serials and psql shows
serials rather than integer columns.