Codechange: Use string_view in IniItem/IniGroup/IniFile.

[PeterN]

Motivation / Problem

I'm still learning. A while back I switched the Ini file system from char * to std::string, not giving any consideration to std::string_view. However after reviewing it, I can see there are places where std::strings are constructed from char *. These temporary strings are then either thrown away, or copied again, and... thrown away.

Description

Use std::string_view in IniItem/IniGroup/IniFile. This avoids making extra copies of strings.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker