rstantools v2.3.1

• Deprecate init_cpp by @jgabry in #105
• Bugfix for standalone functions under 2.31 by @andrjohns in #110
• Only add Makevars for current platform by @andrjohns in #109
• Suppress false-positive warning for standalone functions by @andrjohns in #111

Full Changelog: v2.3.0...v2.3.1