[racket] db library in core

The database library formerly known as (planet ryanc/db) has now
moved into the core as the db collection. Here's the description
from the old package blurb:

   A database interface for functional programmers.

   This library provides a high-level, functional interface to
   PostgreSQL, MySQL, and SQLite databases as well as ODBC data
   sources.  PostgreSQL and MySQL support is implemented in pure
   Racket.  The optional SQLite and ODBC support requires the
   appropriate native client libraries.

I'll continue to update the PLaneT package with bug fixes, at
least until the next release, but new development will happen in
the core version. There are already a few new features (eg,
list-tables) and some organization simplifications (eg,
util/connect merged into the main module), and more are on the
way. A snapshot of the documentation is here:

   http://www.cs.utah.edu/~ryan/tmp/doc/db/

The Continue tutorial has also been revised to use the new db
library (see Section 15):

   http://www.cs.utah.edu/~ryan/tmp/doc/continue/

Feedback and feature requests are welcome. I'm particularly
interested in hearing from people who deal with date/time values
or geometric values, as the current support for those is pretty
rudimentary.

Ryan