Added the set() function for more compact set creation

Hi! I created this as a request on rt.cpan.org a while back, but nothing occurred, so I thought it might be better if I approached you here.

This change adds a function `set()`, which can be exported if requested, that makes a more compact way to create sets. In some of my code, I have a lot of set comparison happening, and I think it makes the code more readable to do something like

        set(1, 2, 3)->is_subset(set(1, 2, 3, 4, 5, 6))

instead of

        Set::Tiny->new(1, 2, 3)->is_subset(Set::Tiny->new(1, 2, 3, 4, 5, 6))

This function isn't provided unless explicitly requested, so it doesn't have any effect on existing code use.

Thanks,
Ricky