Fix issue 16642: byCodeUnit doesn't work properly with alias this.

If a template is going to allow implicit conversions, it really needs to
force the conversion in order to avoid subtle bugs. This fixes
byCodeUnit so that it forces the conversion to a string type for types
that convert implicitly. It also fixes it so that types which implicitly
convert to a string type but are also ranges of characters without the
conversion will not be converted by byCodeUnit.