map::at() may return incorrect item

[Randalphwa]

Migrated from the original repository
Original issue: serge-sans-paille/frozen#156
Originally reported by: @mikekaganski

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map::at() is implemented using at_impl, which itself is:

  template <class This, class KeyType>
  static inline constexpr auto& at_impl(This&& self, KeyType const &key) {
    auto where = self.lower_bound(key);
    if (where != self.end())
      return where->second;
    else
      FROZEN_THROW_OR_ABORT(std::out_of_range("unknown key"));
  }

So whenever lower_bound succeeds, it returns the found value. But lower_bound returns whatever compares not less than — so a check is needed that it is equal to? Possibly using find_impl.