Thinkfan underwent some major changes and the config file
is now based on YAML. This commit contains a number of changes:
- rewrite the module to output the new format;
- add a `settings` option, following RFC 0042[1];
- add fancy type-checking for the most critical options
- use upstream systemd units (which fix the resume issue)
[1]: https://github.com/NixOS/rfcs/blob/master/rfcs/0042-config-option.md
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
