Vector instead of IntMap for running AGs

I replaced IntMap type by Vector to implement running AGs (only for the
hybrid representation).

Result: noticeable reduction in the overhead
Vector implementation + improvements

I used Vector for the implementation of running AGs on simple graph
representations as well. No noticeable improvement, though.

I improved the implementations for the hybrid graph representation. The
runtime is now considerable better for both full sharing and full
unsharing compared to the implementation on the simple graph
representation. The implementation using Vector performs better on fully
shared input (compared to the IntMap-based implementation); and vice
versa the IntMap-based implementation performs slightly better on
unshared input.