Fail if duplicate keys are present

[Ahajha]

#175
This should at least improve the error message. It's not perfect, but at least the cause of the error should end up somewhere on the screen, and should avoid the naive O(n^2) complexity for checking all keys against each other.

I added the following test locally:

TEST_CASE("duplicates") {
  [[maybe_unused]] constexpr auto map =
      frozen::make_unordered_map<frozen::string, int>({{"ha", 1}, {"ha", 2}});
}

This gives the following on clang 15:

[build] /home/alex/Documents/frozen-1/tests/test_unordered_map.cpp:283:35: error: constexpr variable 'map' must be initialized by a constant expression
[build]   [[maybe_unused]] constexpr auto map =
[build]                                   ^
[build] /home/alex/Documents/frozen-1/include/frozen/bits/pmh.h:102:7: note: non-constexpr function 'check' cannot be used in a constant expression
[build]       check("Duplicate keys present, check your input data");
[build]       ^
...

This error message only works on the unordered containers, which makes sense since this change is in hashing code. The non-unordered variants (map, set) don't suffer from the cryptic template depth error anyways, so nothing there to fix really.

[serge-sans-paille]

I like the idea. A few comments though

[serge-sans-paille]

• This should use the KeyEqual parameter.
• This would probably be better if key(items[i]) were saved in a temporary variable.
• move this loop to an helper function?
• Move the check under ifndef NDEBUG ?
• Instead of calling (exit), what about

+      extern void check(const char[]);
+      check("Duplicate keys present, check your input data");

[Ahajha]

I agree with all of these except maybe the ifndef NDEBUG comment. For my use case, I'm writing a library where the keys will end up being supplied by the user. I think it would be better for their experience if they always saw a helpful message, even in release mode. Perhaps we could add a macro that would force the check to happen, but by default it's only in debug mode? So something like #if !defined NDEBUG || defined FROZEN_LETITGO_ENABLE_DUPLICATE_KEY_ASSERTIONS? (we could also default that variable to be defined with NDEBUG, and the user can manually enable it, which simplifies the check)

[Ahajha]

Also, if adding another preprocessor directive, and also since hopefully the old error message should go away in debug mode, should I update the README to mention the new macro?

[Ahajha]

Just some running commentary: Extracting that check to a separate function might be more trouble than it's worth, due to the inputs it ends up needing to be heavily templated and type annotated. I'll try to get it working just to show what it would look like.

I'll need to pass KeyEqual down the stack quite a bit - though this is for correctness so that's fine.

[Ahajha]

I've incorporated everything except the NDEBUG comment, once we reach a consensus I'll add that in.