For a given `template<class...> Target` and list of types `T...`
the alias `UniqueTypes_t<Target, T...>`  refers the instatiated
template `Target<S...>`, where `S...` is generated from `T...`
by removing duplicate types.

This is useful for `std::variant`, because duplicate types
are not fully supported by some implementations. If your
type list is generated by whatever mechanism where you don't
have control over uniqueness, then `UniqueTypes_t<std::variant, T...>`
is what `std::variant<T...>` should have been.

This would fit into `typetraits.hh`. However, this would lead
to a cyclic dependency of `typetraits.hh` and `std/type_traits.hh`.
Hence I've put it into `typelist.hh` which conceptually also
deals with type list, altough no `TypeList<...>` shows up
in the interface of `UniqueTypes_t`.