5.3. Soft DFS

PySol board No. 980662 recursed into a depth of over 3000. On Win32, this caused a stack overflow which resulted in an ugly segfault.
I decided to implement a Soft-DFS scan which does not utilize procedural recursion but rather its own dedicated stack.
This turned out to have an O(1) suspend and resume time instead of O(d) for hard-DFS.
(I later on discovered that a Win32 program can be compiled with more stack space, but I still think Soft-DFS is a better idea.