[mp] Fix count (and other algorithms) API, apply_t<trait, args...>

[GuillaumeDua]

Or strict count: no pack specialization.

Motivation

using one = std::tuple<A, B, C>;
using two = std::tuple<B, C>;
count_v<A, one, two>; // what if we wanna check if "A" is either `one` or `two` ? (ambiguous)

Solution: plain + apply_t wrapper

---

template <typename T, typename ... ttps>
struct count : std::integral_constant<std::size_t, (std::size_t{0} + ... + std::size_t{std::is_same_v<T, ttps>})>{};
template <typename T, template <typename...> typename pack, typename ... ttps>
struct count<T, pack<ttps...>> : count<T, ttps...>{};
template <typename T, typename ... ttps>
constexpr std::size_t count_v = count<T, ttps...>::value;