Implement support for FreeBSD (#766)

* WIP FreeBSD support

* Implement get_cpu_data_list for FreeBSD

* Implement disks for FreeBSD

It doesn't work though as sysinfo doesn't make the device name available.

* Use libxo to read process cpu info on FreeBSD

* Populate get_io_usage with libxo too

Actual I/O stats still aren't populated though as there's not an
easy source for them.

* Share more processes code between macos and freebsd

* Extract function for deserializing libxo output on FreeBSD

* Implement filtering of disks in FreeBSD

* Clean up memory data collection

* Update module docs

src/utils/error.rs

@@ -49,6 +49,7 @@ impl From<std::io::Error> for BottomError {
     }
 }
 
+#[cfg(not(target_os = "freebsd"))]
 impl From<heim::Error> for BottomError {
     fn from(err: heim::Error) -> Self {
         BottomError::InvalidHeim(err.to_string())