I wrote the old interface before playing much with SQLite. Now that I've
played around with it a bit, I found many ways to make the interface more
pleasant and fool-proof:
* it opens the database in a mode that honors foreign keys and
returns extended result codes.
* it forces locking to avoid SQLITE_BUSY and
sqlite3_{changes,last_insert_rowid} race conditions.
* it supports named bind parameters.
* it defers some errors until Step() to reduce caller verbosity.
* it automatically handles calling reset, which was quite easy to forget.
* it remembers the Step() return value, which makes the row loop every so
slightly more pleasant.
* it tracks transaction status.
