constexpr function to && together many bools.
FORMATTER_TYPE now uses __VA_ARGS__.

This avoids the syntax errors caused by commas in the type, which are
common for template instantiations for instance.
(The new) request objects can now be parsed as any object.

This is implemented with a custom partial specialization of
find_formatter. It's only activated though if the object in question is
a boost::variant of request objects, which are checked using a dummy
member type called request.

It's awesome.