Code hierarchy cleanup

[MilesCranmer]

Right now the code dependency graph is pretty complex:

(You can see instructions on the README for generating this)

I think this complexity can be greatly reduced, with the dependencies made more hierarchical. Core.jl is one such example.

For example, I think the following files can be grouped, and made into combined modules:

1. EquationUtils.jl, EvaluateEquation.jl, EvaluateEquationDerivative.jl, SimplifyEquation.jl, and InterfaceSymbolicUtils.jl
2. HallOfFame.jl, Population.jl, and PopMember.jl
3. Mutate.jl and MutationFunctions.jl
4. RegularizedEvolution.jl and SingleIteration.jl

The tricky part will be ensuring that there is still a clear hierarchy. I think the above order is correct, in that i should not depend on i+1, but we might need to move functions around so that the dependency structure is still satisfied.

If anybody is interested in working on this, let me know.

[MilesCranmer]

1. Is currently being grouped into its own module here: https://github.com/SymbolicML/DynamicExpressions.jl.