Use low-level contiguous_sequence optimisations where appropriate about flux

[tcbrindle]

@DeveloperPaul123 Not only is it reasonable, it's essential: a memcmp optimisation of equal must require that both sequences have the same type, among other conditions.

[tcbrindle]

@DeveloperPaul123 Thinking about it some more, I think it's probably better to not use memmem for now and stick with portable standard library facilities... we can look at platform/vendor-specific optimisations later if/when they're needed.

[tcbrindle]

Added in #103, thanks @DeveloperPaul123!

[DeveloperPaul123]

I've started work on this in #103

[DeveloperPaul123]

@tcbrindle I have a question while working on equal()

There is this test case:

 // Different but comparable element types
{
    int arr1[] = {1, 2, 3, 4, 5};
    float arr2[] = {1.f, 2.f, 3.f, 4.f, 5.f};

    STATIC_CHECK(flux::equal(arr1, arr2));
}

It seems that the intension is to compare values and accept them as equal. If we want to optimize with std::memcmp, does it seem reasonable to require the the values of the 2 sequences be of the same type? Currently, using the std::memcmp optimization causes the above test to fail (where it previously passed).

[DeveloperPaul123]

@tcbrindle Currently, std::memmem is a GNU extension. Is it desirable to use this on platforms that's it's available on? Currently C++ extensions are explicitly turned off via CMake