Include test to verify patch that removes 'using namespace std' from headers

[ekilmer]

A patch was generated (#143) to remove the using namespace std from the sleigh headers due to issues with compilation on Windows (#139).

This repo should include a test that will detect (in some way) whether the rebased patch continues to work.

Compiling the project and running the tests is a good indication that the patch continues to work, but if a new header file is added and includes a using namespace std; that we forget to remove, then we could have regressions.

It's a bit unclear to me what the test would look like, but maybe somebody has a better idea.

[mrexodia]

Perhaps something like this could work?

struct vector {};
struct set {};
struct unordered_map {};
struct string {};

// not sure if there are any other public headers?
#include <sleigh/libsleigh.h>

int main() {
vector v;
set s;
unordered_map m;
string str;
}

For MSVC it should use use target_compile_features(test PRIVATE cxx_std_20) because there were some weird issue with /permissive- and std::byte colliding deeply in the sleigh headers.