Eliminates lots of overhead.

Split the string into path segments with StringView::Split().  This
prepares to eliminate all allocations from the method.

std::all_of becomes constexpr in C++20. I'm not sure it results in better
performance.

Found with useStlAlgorithm
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The _GLIBCXX_USE_C99_MATH macro is defined in glibcxx by c++config.h, which
gets included by every header. Which means a header needs to be present.

Use std::string_view to avoid modifying the string for the temporary
null terminators.