Remove unnecessary import
Improve create CaseTypes, SatisfiedTypes functions

- Variadic
- Simplified

See #64; for #19.
Add create ExtendedType, ClassSpecifier functions

(And ClassInstantiation, but I might unshare that, it’s really just
auxiliary for the other two – I don’t see why you’d want to use it
directly.)

For #19.
Add tests for create ExtendedType, ClassSpecifier functions