Enable iteration over just types (not values) with Hybrid::forEach()

Jö Fahlkerequested to merge
feature/meta-type-v2 into master

This makes it possible to iterate over a list of types without the need to create objects of those types. Example:

using namespace Hybrid;
forEach(TypeList<void, NonConstructible, int>{}, [] (auto metaType) {
  std::cout << className<typename decltype(metaType)::type>() << std::endl;
});

This comes from our internal module dune-react, where I use this to iterate over a list of grid manager types.

This is a follow-up to !210 (closed). The difference is that this now uses the existing type TypeList as the list of types. It still uses MetaType<T> to wrap a single type T, and TypeList<T...> has been changed to an alias of std::tuple<MetaType<T>...>.

Closes: !210 (closed).

Planned merge: 2017-05-12

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