OCaml 4.12.0

• Major progress in reducing the difference between the mainline and
  multicore runtime
• A new configuration option ocaml-option-nnpchecker which emits an alarm
  when the garbage collector finds out-of-heap pointers that could cause a crash
  in the multicore runtime
• Support for macOS/arm64
• Mnemonic names for warnings
• Many quality of life improvements
• Many bug fixes

See the detailed list of changes