Codechange: Emplace std::pair instead of creating then inserting.

[PeterN]

Motivation / Problem

In various places we create a std::pair, then call std::map's insert, or push it onto a std::vector. This may create two pairs, one as a local temporary, which is then copied into its resting location.

Description

Both std::map and std::vector have emplace functions which avoid this and create the pair in-place, so use them.

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