Guard against large capacities when allocating

[rtheunissen]

We currently allow allocation via allocate(int $capacity), but we don't check for overflow or have a maximum capacity. @nikic also mentioned that we should be using the safe_* variants when multiplying sizeof by a variable capacity.

[rtheunissen]

2.0 won't support allocate so this is now a 1.* issue.

[rtheunissen]

I think we should just remove explicit allocation entirely, in both 1.x and 2.0