Add IsProductType and IsSumType

[kleinreact]

The PR adds some type based indicators to BitPack that allow to check whether the instance type is a sum- or product type. This variant uses type-level Bools to communicate the property.

An alternative purely constrained based implementation is given by #3031.

Still TODO:

• Write a changelog entry (see changelog/README.md)
• Check copyright notices are up to date in edited files

[martijnbastiaan]

This won't be necessary anymore because #3029 (comment).