Macro map_errno 

macro_rules! map_errno {
    {
        $err_ty:ty { $($errno:pat => $err:expr),*$(,)? }
        $(inherit $base:ty;)?
    } => { ... };
}

Generate an implementation of FromErrno for the given type.

Example:

map_errno! {
    GpsError {
        EAGAIN => Self::StillCalibrating,
    }
    inherit PortError;
}