Improve "native" support for date and other date/time types

[eyalroz]

We currently support, in principle, two of MonetDB's date/time-related types: "date" and "daytime"; but even that's limited; we're not even dumping these values reasonably in our example program.

We should:

1. Ensure appropriate C++'ified definitions of these types are available.
2. Add support for the "timestamp" and "tzone" types.

Notes:

• For "date" specifically, values are represented as days since the epoch, which is Jan 1st in year -5867411, and skipping year 0 (yes, a little weird).
• There's a bunch of code we can use in mtime.h and mtime.c which are already part of the repository.

Question: Should we create proxies for / provide makers of std::chrono types, or just provide our own custom types and let the user worry about conversion?