Integrate problem changes from PR draft #173

[Saethox]

It was a bit trickier than I expected to entangle the problem definitions from the new component system in #173, but it does compile.

This is the first PR of 3-4 PRs to integrate #173 fully. It only focuses on the problems, utils and population modules, as the components, framework, conditions, and heuristics, and state modules will be part of the next PR.

Changes

• Remove old problems in accordance with #172, #169, #155
• Remove build.rs and gitlab-ci.yml, as not needed anymore
• Update Cargo.toml (the yet unused packages will be used in the next PRs)
• Update README with tech report
• The ObjectiveFunction trait is a simpler version of Evaluator for problems where the objective function can be easily specified, and Sequential<P> and Parallel<P> offer a default implementation of a sequential and parallel Evaluator, respectively, when P implements ObjectiveFunction
• Simplification and refactoring of the additional problem traits, now with a proper hierarchy (e.g. LimitedVectorProblem depends on VectorProblem, etc.) and a separate TSP trait to access distance information
• Added TryDefault for the future
• Added much more extensive documentation to the new modules

Postponed Changes

The following changes had to be postponed because they are too cumbersome to integrate without the new component system:

• The Evaluator component (now PopulationEvaluator) was changed from accessing the EvaluatorInstance in the State to having a T: Evaluator type parameter
• This makes it possible to have multiple Evaluators in the state at the same time, and decouples Evaluator and Problem somewhat
• Problem::default_evaluator was removed in favor of a TryDefault bound on Evaluate

[Saethox]

Just one question: Do we have enough documentation in the MAHF readme and the problem repo readmes that everybody knows how to use it?

@HeleNoir There is still room for improvement, I'll look into it in the next PR. For example, #111 would be nice to have.

[Saethox]

@luleyleo Are we ready to merge, then?