WIP: a class for a resiable array of long IDs.

IDSet is made to hold a set of positive long IDs.
For performance and memory usage reason, it wrpas a primitive long
array, and resize it (using copy) in case the initial capacity is
exceeded.
It maintains a sorted array, to speed up add, search and remove
operations. It is a set, and does not permit duplicate values.

The current design violates about any good programming practices,
exposing its fields as public, and referecing input arrays. Again all
for the sake of performance and memory usage. We will have to be extra
cautious with this, as this class will be part of the public API.

Indeed this class is intended to store the IDs of the spots that are
visible, etc... in a certain frame. Users will have to access it to
iterate over these IDs.
Signed-off-by: Jean-Yves Tinevez <jeanyves.tinevez@gmail.com>