Use Unicode-aware implementations of LOWER() and UPPER() in SQL queries (#865)

Fixes #840 by creating application-defined SQL functions
(https://www.sqlite.org/appfunc.html) for Unicode-aware implementations
of `LOWER()` and `UPPER()`. This uses
`String.prototype.toLower/UpperCase()` JS method.

I initially wanted to just redefine `LOWER()` and `UPPER()` but due to
[sql.js not supporting the definition of deterministic
functions](https://github.com/sql-js/sql.js/issues/551), I had to just
define them as separate functions and use that in the appropriate
places. It's probably better like that anyway...