Move default history location to ~~state/

[nbraud]

• Use state directory as the default history_path, see Store history in application state directory #35
• Automatically migrate history from ~~/ to ~~state/
  Setting history_path to the old value will prevent automatic migration
  Tested on Linux (NixOS)

[nbraud]

I considered what happened if ~~/ and ~~state/ are the same:
in that case, file_info(old) and file_info(new) are either both non-nil or both nil,
so the migration path is not taken either way.