Add closures support for compilation speed

[AntonReinhard]

Also removes the use of RuntimeGeneratedFunctions inside the generated function itself.

This adds the use of closures to reduce compile times.

Still todo:

• Make closures more optional, off by default
• Add automatic closure size
• Make closures recursively available so even larger functions can be generated
• Improve function generation performance, currently it is very slow, likely because of heavy usage of sets
• Investigate how to improve the runtime performance of the generated functions when using closures. The current problem is likely too many allocations.

Fixes #33

[AntonReinhard]

The rest of the todo items are not immediately necessary or depend on a more powerful FunctionCall object. I will postpone this to later PRs.