status_code_domain::string_ref and atomic_refcounted_string_ref have a bad API around zero termination

[Niall Douglas]

• The caller is expected to malloc a region length + 1 and zero terminate it before handing it in.
• This is highly prone to mistake and surprise.
• The API should be refactored around what path_view does i.e. caller must specify zero termination.
• I also think caller should not be calling malloc in the first place, that's an internal implementation detail which should not be exposed.
• Also what about on Freestanding where there is no malloc?