Move problem to submodules

[Qwebeck]

Currently state search and rl are sharing problem implementations.
It has a drawback that problems would not be synchronized between repositories.
Fortunately, git allows to solve this problem by creating a submodule.
It will work in a way, that every repo using that submodule, will contain a reference to the repo with that submodule, so end user will be able to pull changes from it.
The concept is fairly simple, but useful, so i suggest to utilize it here.