Fix conversion warnings in CvtFormat

[mrotteveel]

• adds some convenience methods for std::string_view in StatusVector and string

[mrotteveel]

@asfernandes My reason for using a PR are:

1. is it OK to add these methods accepting std::string_view?
2. Should I add more complete support instead of just those two methods I needed to solve my immediate problem?

[mrotteveel]

Also replaced a use of const string_view& I did earlier.