Standard implementation for lower_bound/upper_bound/equal_range.

[nbrachet]

Namely (quoting cppreference.com):

• lower_bound "Returns an iterator pointing to the first element that is not less than (i.e. greater or equal to) key."
• upper_bound "Returns an iterator pointing to the first element that is greater than key."
• equal_range "Returns a range containing all elements with the given key in the container. The range is defined by two iterators, one pointing to the first element that is not less than key and another pointing to the first element greater than key."

[adamshapiro0]

@nbrachet this is great timing - we just ran into the same issue and I was about to make this change myself when I found your PR. I suggest adding some improved test coverage as follows:

  auto lower_bound = ze_map.lower_bound(0);
  REQUIRE(lower_bound == ze_map.begin());

  lower_bound = ze_map.lower_bound(5);
  REQUIRE(lower_bound == ze_map.begin());

  lower_bound = ze_map.lower_bound(20);
  REQUIRE(lower_bound == ze_map.begin() + 1);

  lower_bound = ze_map.lower_bound(25);
  REQUIRE(lower_bound == ze_map.begin() + 2);

  lower_bound = ze_map.lower_bound(30);
  REQUIRE(lower_bound == ze_map.begin() + 2);

  lower_bound = ze_map.lower_bound(40);
  REQUIRE(lower_bound == ze_map.end());

  auto upper_bound = ze_map.upper_bound(0);
  REQUIRE(upper_bound == ze_map.begin());

  upper_bound = ze_map.upper_bound(10);
  REQUIRE(upper_bound == ze_map.begin() + 1);

  upper_bound = ze_map.upper_bound(15);
  REQUIRE(upper_bound == ze_map.begin() + 1);

  upper_bound = ze_map.upper_bound(30);
  REQUIRE(upper_bound == ze_map.end());

  upper_bound = ze_map.upper_bound(40);
  REQUIRE(upper_bound == ze_map.end());

[adamshapiro0]

Oh and actually one more test change to test for the at() fix:

   SECTION("Lookup failure") {
+    REQUIRE(frozen_map.find(3) == frozen_map.end());
+    REQUIRE_THROWS_AS(frozen_map.at(3), std::out_of_range);
     REQUIRE(frozen_map.find(5) == frozen_map.end());
     REQUIRE_THROWS_AS(frozen_map.at(5), std::out_of_range);
   }

[nbrachet]

Thanks for the suggestions. Added in https://github.com/serge-sans-paille/frozen/pull/125/commits/c445fef5a4c8422a521575c7ee3aa54727322edf

[serge-sans-paille]

Thanks for this fix! Can you rebase it against master branch?