frozen::unordered_multiset?

[Randalphwa]

Migrated from the original repository
Original issue: serge-sans-paille/frozen#114
Originally reported by: @davidson16807

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What would be required to create the frozen equivalent of unordered_multiset?

I'm asking since I think the existence of such a thing could allow for creating nice and easy compile-time unit libraries akin to mpusz/units or benri:

constexpr frozen::unordered_multiset<frozen::string, int, 1> mass = {{"mass", 1}};
constexpr frozen::unordered_multiset<frozen::string, int, 2> acceleration = {{"length", 1}, {"time", -2}};
constexpr auto force = combine(mass, acceleration);

Personally, if I'm writing a software framework that relies on a dimensional type system I'd much rather rely on a low lying abstraction like a frozen::unordered_multiset rather than force users to add one of these units libraries to their list of dependencies.