Steppable versions of A* (`astar_reach`) and of other algorithms

[mikwielgus]

Hi,

It would be nice to be able to run A*, and possibly some more algorithms, e.g. idastar, iddfs, step by step, in an external loop. You can already do that for some algorithms using the dijkstra_reach, bfs_reach, dfs_reach functions. But currently, A* doesn't happen to have that, you can't anyhow break out of A* execution, you have to wait for it to finish. Same for a few other algorithms in this codebase.

Being able to step an algorithm allows creating an animated visualization of its operation, which can be very useful for inspecting its behavior.