Function to_string

include/boost/histogram/axis/ostream_operators.hpp:22–22

namespace detail {
inline string_view to_string(const transform::identity&) { return {}; }
inline string_view to_string(const transform::log&) { return {"_log", 4}; }
inline string_view to_string(const transform::sqrt&) { return {"_sqrt", 5}; }