Add tutorials for type-erasure, reflection and vanishing-this

[jbcoe]

I'm not entirely sure that these tutorials are necessary.

The act of writing them was useful and perhaps they will be useful to readers of the (increasingly novel and technical) code in #95.

[Twon]

I think these are great, first 2 down and working through the 3rd now, but these are definitely useful (I'm learning reflection from the 3rd).

Perhaps the files could be renamed with a number indicating the order of the tutorials?

1. 1_type_erasure.cc
2. 2_vanishing_this_pointer
3. 3_reflection.cc

The pedant in me finds it odd to start in the middle, go down and then go up in the file view in VSCode.

[Twon]

May as well just use std::vector's range constructor

Suggested change

	consteval std::vector<std::meta::info> data_members_of(std::meta::info type) {
	consteval std::vector<std::meta::info> data_members_of(std::meta::info type) {
	return std::meta::nonstatic_data_members_of(
	type, std::meta::access_context::current());
	}

[Twon]

consteval std::vector<std::meta::info> member_functions_of(std::meta::info type) {
    // is_function alone would also match Widget's implicit special
    // members (default constructor, destructor, ...);
    // is_special_member_function excludes those, leaving just the
    // two ordinary methods below.
    auto const is_member_function = [](std::meta::info member) {
        return std::meta::is_function(member) && !std::meta::is_special_member_function(member);
    };

    return std::meta::members_of(type, std::meta::access_context::current()) |
           std::ranges::views::filter(is_member_function) |
           std::ranges::to<std::vector<std::meta::info>>();
}