Add optional<T> data type to supported types

[MiroPalmu]

Have optional to be part of vat variant, where T can be int32_t, uin32_t, int64_t, uint64_t or std::string.

Example code using this feature:

auto input_a = std::optional<int>{ };
auto input_b = std::optional<std::string>{ };

const auto options = {
    { { "a" }, input_a, "Input a"},
    { { "b" }, input_b, "input_b"}
};

argz::parse(about, opts, argc, argv);

if (input_a) foo(input_a.value());
if (input_b) bar(input_b.value());

Notice that T can not be bool due to ambiguous meaning in the following example:

auto bool_opt = std::optional<bool>{ };

const auto options = {
    { { "a" }, bool_opt, "Input a"}
};

argz::parse(about, opts, argc, argv);

// Now if command line is: "./prog -a"
// should input_a be empty or contain true?

if (input_a) foo(input_a.value());

If optional would contain integer or string input_a would be empty optional.

[MiroPalmu]

Force push was to remove redundant construction of std::reference_wrapper which got implicitly converted to normal reference immediately.

[stephenberry]

@MiroPalmu
This is brilliant! I made a tiny fix to build with clang. But, thanks so much for this addition.

[MiroPalmu]

@stephenberry

I realized one complication about this. Templated/generic lambdas were added in c++20 and CMakeLists.txt at the moment has target_compile_features(argz_argz INTERFACE cxx_std_17)

This was not apparent when I was testing this because CMake somehow selects c++20 automatically for me.

[stephenberry]

@MiroPalmu
Ah, thanks for pointing this out. I'm fine moving to a C++20 requirement. All my other projects are moving to this requirement and I find C++20 to be widely available now. I'll update accordingly.