Update gsl-lite dependency to v1.0

[mbeutel]

This PR bumps the gsl-lite dependency version to v1.0.

gsl-lite v1.0 has some breaking changes (cf. our v1.0.0 release notes), which necessitate some changes here.

The most obvious changes are the name of the header file (now <gsl-lite/gsl-lite.hpp>) and the namespace (now gsl_lite; I added a gsl alias to namespace telnetpp).

By default, span<> does not define comparison operators anymore, which means that we need to compare spans through other means. I added a bytes_equal() function object to explicitly compare two byte spans and adapted the code where necessary. A less invasive fix would be to re-enable span comparison operators by defining gsl_CONFIG_ALLOWS_SPAN_COMPARISON=1. However, comparison operators are no longer provided by default for good reason (unclear semantics), and C++20 std::span<> doesn't have them either, so this appears to be the most forward-looking way to handle it.

Some minor adjustments were needed because span<>::iterator is now a custom iterator class rather than a plain pointer type.

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This change is 

[mbeutel]

The Appveyor tests fail because they retrieve their dependencies with Vcpkg, which doesn't have gsl-lite v1.0 yet. I submitted a PR to upgrade gsl-lite to v1.0.0, only to find that this breaks Telnet++. A Catch-22 situation :)

[KazDragon]

@mbeutel This is great, thanks for the contrib. I'm actually in the (slow) process of removing gsl-lite from my libraries because C++20 obsoletes the parts that I use, but this is very fine for the situation that it's in and if it fixes integration with vcpkg, then all for the better. Much appreciated.

[mbeutel]

Excellent, thank you for the quick response!

The GSL was already a stop-gap when the Core Guidelines were introduced 10 years ago; it really shouldn't need to exist anymore. C++20 added spans (albeit unchecked ones), and C++26 will finally add contracts, so more and more of the GSL will become obsolete. I think gsl-lite can be considered "done" at this point; I just wanted to finally resolve the interference issue with Microsoft GSL, which in turn required the breaking change.

[KazDragon]

In any case, your contributions have been a great help, so thank you.

[mbeutel]

Thanks for preparing the new release so quickly. Telnet++ 4.0.0 is now available via Vcpkg.

[KazDragon]

Awesome!