Add support for regular types

[jeltsch]

We shall add support for regular types. Concretely, we shall do the following:

• Add the following components:
  • A subclass NontrivialUniversal of Universal with methods that turn values of base types into nontrivial universal functor values and have default implementations that reflect the sum-of-products views of the respective base types
  • A class Regular of regular types with a method that turns values of regular types into nontrivial universal functor values and has a default implementation for instances of SOP.Generic
  • All instantiations of the class Regular
  • A method regular of the class Universal that turns values of regular types into universal functor values and has a default implementation for nontrivial universal functors
• Adapt the final section of README.md and the description of the Control.Modal module.