SQLite support

[andy-hanson]

demo/db.crow uses LMDB. That was easy to port to crow due to having a simple API, but it's not as popular as SQLite, so it would be nice to have an SQLite demo too.

The task would be to write a simple program like demo/db.crow that uses SQLite instead of LMDB. This would involve:

• Transcribing needed parts of /usr/include/sqlite3.h to a new file in this repository, include/sqlite3.crow.
• Writing a wrapper library include/crow/db/sqlite3.crow; compared to the underlying include/sqlite3.crow, this should be safe, and use crow types like str instead of C-like types like char*.
• Write demo/sqlite.crow to show off the library.