Move require_once out of functions?

It's really a minor issue, but having `require_once 'Expressions.php';` inside `build_insert()` and `apply_where_conditions($args)` of SQLBuilder.php seems a little odd, as that requires a stat of Expressions.php everytime those functions are called.  It's a sub-microsecond optimization, but my framework is already running in the sub-microseconds for most requests.  I don't see anything that'd be harmed by letting the autoload handle loading it, or even just moving it to the top of the file, so it's always loaded, but I'm still a little new to the guts of phpActiveRecord.