Make WITHOUT TIME ZONE the default for TIMESTAMP and TIME data types.

 This is a big change from past behavior, but the last release was
 designed to handle this correctly for dump/restore upgrades.
Fix up handling of SET value arguments. Allow lists for most options at
 least at the parser level; multiple values may be rejected at the
 command processor of course.
Allow more variations on values for SET commands, including integer and
 float values where formerly stringy fields were required.
Check precision specification for date/time fields against the true
 precision range allowed by the data types. Especially useful with the
 new int8-based storage for these types, where precision is fixed and
 predictable.
Stub out a basic CREATE ASSERTION per SQL9x. Does not do anything (yet) but
 should be augmented as appropriate.
Minor fixups in braces and tabbing.