add support for toml::get<std::unordered_set>

[evanwporter]

Easier than calling toml::get<std::vector<T>> then having to convert it to std::unordered_set

As part of the PR I added a couple of basic tests. They are mostly for demonstrating that this works.

[evanwporter]

@ToruNiina

I though about adding a is_set_like condition that copies is_array_like with the exception of the push_back (instead it requires insert) but I thought it might be too broad, since for example std::vector support insert but with different parameters.

Also it might be good to add a condition for std::set.

[ToruNiina]

std::unordered_map is a standard library container, so it is well-known enough to introduce a dedicated meta function. I have no complaint about this approach. The same applies to std::set. Adding std::set sounds good. But for now, I will merge this as it is.

Thank you!

[evanwporter]

@ToruNiina

Yes! std::unordered_set is just something I need in my implementation of toml11, but I'll create a PR for std::set when I have time

[evanwporter]

@ToruNiina

I was thinking about it and it may be better to implement an is_set_like trait. This way we can cleanly include std::set and std::unordered_set. This will also support something like ankerl::unordered_dense::set.

The conditions for is_set_like can be as follows:

• T has an iterator
• T has a value_type
• T supports insert(x)
• T is not toml::value::array_type
• T is not std::string
• T is not std::string_view
• T is not map-like
• T does not have from_toml() member function
• toml::from is not defined
• A constructor from toml::basic_value is not defined
• T has key_type

The distinguishing part here is that sets have the key_type.

Not urgent, just something to think about.