Enable constexpr support for fmt::format (fmtlib#3403)

[msvetkin]

This PR allows to use fmt::format at compile-time by:

• Adding FMT_CONSTEXPR_STRING macro
• Mark format/to_string with FMT_CONSTEXPR_STRING
• Add a few fmt::format compile-time test-cases

[vitaut]

Thanks for the PR!

[vitaut]

Why does it depend on consteval?

[msvetkin]

FMT_USE_CONSTEVAL enables FMT_CONSTEXPR20 which is required.
I could add comment or introduce something FMT_USE_CONSTEXPR20

[vitaut]

Do we need a lambda here?

[msvetkin]

Yes, unfortunately we do need it. The issue is that all allocated memory at compile time should be deallocated.
That means we are not able to write like this:

constexpr auto s = fmt::format(FMT_COMPILE("{}"), true)); // leaks allocated memory at compile-time to runtime.
EXPECT_EQ("true", s);

Alternatively we could use static_assert:

static_assert(fmt::format(FMT_COMPILE("{}"), true) == "true");

[vitaut]

Let's keep the lambda for now but I don't think we need to test different arguments since this is already tested elsewhere. What we need to test here is that fmt::format can be called at compile time so I suggest picking on type of an argument (e.g. int) and testing it.

[msvetkin]

I have replaced boolean test cases with int and drop test case for a type with user provided formatter.

[vitaut]

defined(__cpp_lib_constexpr_string) seems redundant since we only enter this branch if !defined(__cpp_lib_constexpr_string) is false.