Hppc allocated[] removal pull request

Here is a port of the HPPC-RT modifications removing the allocated[] array.
Note I just made the bare minimum to make it pass the existing tests.
Also the iteration compatibility is somewhat "broken" from before: lslot() returns -2 on the default key,
and cursor.index = -1 for the default key. The direct iteration is also somewhat more complicated now, since you have to check the special default key allocation flag in addition to iterating the array.