Adalog: rework Topo_Sort to use recursion

While Topo_Sort was so far using several linear passes over the atoms to
sort, this new version uses a recursive traversal to add dependencies
before the atom that depends on them.

This will make it possible to handle atoms that have several
dependencies. We had a satisfying workaround for N_Predicate so far as
they can always all go at the end of the sorted list of atoms. An
upcoming change will introduce N_Propagate, which will not only depend
on several logic variables, but will also provide a definition for one,
making the current Topo_Sort unsuitable.

Since Topo_Sort now no longer adds relations with unsatisfied
dependencies to its result (it used to do it for N_Predicate), there is
no need to have the corresponding protections in Solve_Atomic.

TN: SB20-024
For libadalang/langkit#618

Author: pmderodat