Instance-local variables

# Currently:

has $!sth-ensure-item;
has $!sth-list-items;

method ensure-item
{
    $!sth-ensure-item //= $!dbh.prepare(…);
    $!sth-ensure-item.execute(…);
}

method list-items
{
    $!sth-list-items //= $!dbh.prepare(…);
    $!sth-list-items.execute(…);
}

# Preferred:

method ensure-item
{
    something $sth := $!dbh.prepare(…);
    $sth.execute(…);
}

method list-items
{
    something $sth := $!dbh.prepare(…);
    $sth.execute(…);
}