This commit adds a new function transitive_closure_dag() which is an
optimized method for computing the transitive closure for DAGs. In
support of this a new function descendants_at_distance() for finding the
nodes a fixed distance from a given source to both rustworkx and
rustworkx-core.

Related to: Qiskit#704

This commit adds a new function to rustworkx-core for building a
transitive closure inplace out of an input DAG. The function takes
ownership of the input graph and will mutate it to add edtges to make it
a transitive closure before returning it. This is then used internally
by the retworkx python function transitive_clsoure_dag().