constructable from std::optional

[shiz01]

Make tl::optional<U> constructible from std::optional<T>, when T is convertible to U.

[BenFrantzDale]

Ideal would be a way of registering other optional types. We use std::optional and absl::optional. It would be nice if I could do something like template <typename T> struct tl::is_some_optional<absl::optional<T>> : std::true_type {}. Likewise boost::optional. And likewise std::nulloopt_t, absl::nullopt_t, etc.