Add support for incomplete types

[LocalSpook]

This just requires replacing the view base with an is_view type trait that can be queried on incomplete types.

Fixes #3180.

[vitaut]

Thanks for the PR. Overall looks good but let's not introduce a new test binary just for this and instead add a test case to format-test (I don't think we need to test interaction with color).

[LocalSpook]

Done

[vitaut]

Thinking more of it, it's quite convenient to inherit from view. I suggest keeping it by defaulting is_view to a SFINAE-friendly equivalent to is_base_of, something like:

struct view {};

template <typename T, typename Enable = void>
struct is_view : std::false_type {};

template <typename T>
struct is_view<
  T, std::void_t<decltype(static_cast<view&>(std::declval<T&>()))>>
    : std::true_type {};