Implement views::counted

[Michael Schellenberger Costa (miscco)]

Partially addresses #39

Note: Reading the requirement in http://eel.is/c++draft/ranges#range.counted-2 I was lazy and assumed that it is sufficient that the passed argument is convertible. Is that correct or do we need to add some _Choice magic?

[Michael Schellenberger Costa (miscco)]

Casey Carter (@CaseyCarter), I am unsure whether we should test anything more as we essentially tested everything already with the span and subrange tests.

Other test ideas besides checking for content?

[Michael Schellenberger Costa (miscco)]

Urgh, that conversion warning of auto result = ranges::views::counted(Iter{input}, 3); sets me off.

I guess I should have been more careful regarding the convertible_to<iter_difference<_Iter>> requirement

[Michael Schellenberger Costa (miscco)]

Casey Carter (@CaseyCarter) I feel like the convertible_to<_Diff, iter_difference_t<_It>> requirement on views::counted would also make sense for counted_iterator, where we explicitly require the second argument to be of type iter_difference_t<_It>

This was also the reason that in the test we have these two lines in the test:

            counted_iterator<Iter> constructed(Iter{input}, iter_difference_t<Iter>{2});
            counted_iterator<Iter> constructedEmpty{Iter{input}, iter_difference_t<Iter>{0}};

Would you say that this is something we might want to change?

[Michael Schellenberger Costa (miscco)]

Also the actual error is that std::span takes an size_t as second argument, so I assume we would need to cast it to size_t before passing it so std::span even if that might be impossible :(

[Casey Carter (CaseyCarter)]

Not a real review, just correcting the problem from #1393.

[Casey Carter (CaseyCarter)]

Every contribution counts! (But some more than others.)