Simplify the code by using the contains implementation in IntervalMap.
There is a slight change of behavior here: We now treat an allocation of
size 0, as if it was size 1. This guarantees that the returned addresses
will be unique, whereas previously we would allow the allocation
function to return the same zero-sized region multiple times, as long as
it is not null, and not in the middle of an existing interval (but the
situation when we were placing an larger interval over a zero-sized one
was not detected).
I think this behavior makes more sense, as that is pretty much the same
guarantee as offered by malloc (except that is permitted to also return