Allow DATE and TIME types to be specified in the Octo DDL but treated as strings internally

Octo currently lacks any DATE or TIME types. These will eventually be implemented, likely after the 1.0 release. In the meantime, Octo should allow the specification of DATE and TIME types in the DDL so that application-level code can distinguish between them, even if Octo does not.

The most straightforward way to achieve this would be to allow the parser to handle DATE and TIME keywords in the DDL, but handle both "types" internally as VARCHAR, i.e. strings.