map and set initializer_list constructors

[jeaye]

Hello! Thank you so much for making immer. I see that arrays and vectors support initialize_list ctors. Is there any reason why maps and sets don't? I haven't found any docs or issues suggesting a reason, so it could be an oversight?

// This works.
immer::vector<int> v{ 1, 2, 3, 4 };

// These do not compile.
immer::set<int> s{ 1, 2, 3, 4 };
immer::map<int, int> m{ {1, 2}, {3, 4} };

// All of these work.
std::vector<int> v{ 1, 2, 3, 4 };
std::set<int> s{ 1, 2, 3, 4 };
std::map<int, int> m{ {1, 2}, {3, 4} };

[arximboldi]

It is simply a missing feature. I've been wanting to add it when I add map transients, which is top of the TODO-list.