Codechange: make SQString::Create that supports std::string and use that

[rubidium42]

Motivation / Problem

Number of places doing .c_str() to then (implicitly) run strlen to determine the length within the Squirrel API.

Description

Add std::string supporting function to create SQString that passes the string size to the function that actually creates the object.

Limitations

None.

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, gs_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

[LordAro]

changing from c_str to data without any changes to length/size makes me uncomfortable

[rubidium42]

changing from c_str to data without any changes to length/size makes me uncomfortable

1: since C++11 .data() and .c_str() are defined/specified to be the same. The specifications literally says: "The returned array is null-terminated, that is, data() and c_str() perform the same function.".
2: we are already passing the string's size there, unless the size is overridden by the caller. So conceptually .data() and .size() seem better than .c_str() and .size().