feat: add real fitness functions

Add linear, step, rastrigin, griewank and schwefel fitness functions
along with the tests for them.

Try using static vector where appropriate, but use OVector for those
where filling with zeros won't work. This is mostly a constrain for
the tests... rather than a real constrain. In the end the dimensions
will be given for each problem.

tests: move all tests to separate submodules

chore: guard test imports by #[cfg(test)]

chore: split types and functions to separate module files