Fixing unintentional copying, possible index out of bounds; Enable compilation with g++-11

extending std::optional does not apparently work. It is the last straw. I have implemented ext::ostream + ext::ostringstream + ext::ofstream and removed the ext::optional.

I'm hoping to remove more ext::[containers] that don't need to be present and were only present to let ADL find their operator <<. (With the adoption of ranges, more will follow!)