Support std::multimap and std::unordered_multimap

[brodeuralexis]

This commit enables support for std::multimap and std::unordered_multimap using fine::decode and fine::encoder.

This can allow users to handle keyword lists more easily:

std::nullopt_t example(ErlNifEnv *, std::multimap<fine::Atom, fine::Term> opts) {
  // ...
  return std::nullopt;
}

Unlike std::map, std::multimap does not have an operator[], meaning that accessing keys that do not exist will not default initialize its value on access. User must use std::multimap::find and deal with the absence of key.

If accumulating values for the same key is required, std::multimap::range_equal, and std::multimap::lower_bound and std::multimap::upper_bound can be used in conjunction with <ranges> (C++20) or <algorithm> (C++17):

std::nullopt_t example(ErlNifEnv *, std::multimap<fine::Atom, fine::Term> opts) {
  std::vector<int64_t> opts_numbers;
  std::transform(
    opts.lower_bound(atoms::number),
    opts.upper_bound(atoms::number),
    std::back_inserter(numbers),
    [&](const auto term) {
      return fine::decode<int64_t>(env, term);
    });

  // ...
  return std::nullopt;
}

[jonatanklosko]

Tiny comments and looks good to me!

[jonatanklosko]

Thank you!