Remove Sql->select in favour of Sql->select_* functions

$sql->select relies on mutable state on the $sql object to function
correctly. Thus, multiple $sql->select calls do not nest correctly,
which can lead to errors occuring. $sql->select has no benefit over
$sql->select_*, and in fact carries more complexity with it, so I
have removed this function entirely, and changed any previous
references to now call the approriate ->select_* method.