Manually fix up some std::is_same<...> places.

[bangerth]

I was getting some weird error messages about undefined symbols. I did not try to track this down in detail, but I believe that what happened is that the function declaration used std::is_same_v, but the definition continues to use std::is_same. If this is just a namespace-level function, then the compiler will simply treat these as different overloads of the same name -- leading to linker errors.

So I simply went through the entire library and replaced all occurrences of std::same_as by std::same_as_v (except for one place where we used the type, not ::value). The places I had missed previously are the ones where std::is_same appears on a different line than ::value -- hard to catch by script.