Codechange: Don't use macros for DAYS_TILL and friends

[2TallTyler]

Motivation / Problem

We use macros for date lookups.

I needed to change this for my ongoing work with time, but it's no longer necessary. Might as well clean it up, though.

Description

• Convert the macros in date_type.h into inline functions.

Limitations

• I have not measured performance changes.
• There might be better ways to do this.

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