bug#34255: wishlist: Add guix-y way to create swapfile

[swedebugia]

Hi

I specified a swapfile in my config without creating it on disk manually.

(swap-devices '("/swapfile"))

Then I predictably got this error:

guix system: error: exception caught while executing 'start' on service 
'swap-/swapfile':
In procedure swapon: "/swapfile": No such file or directory

I think we should let guix do all the dirty work of generating the 
swapfile so we can do something like:

(swap-file      
        (file "/swapfile"))
        (size "1G"))

And then guix will invoke the relevant file tools to create and enable 
the swap file.

We could add a check to see if the file exist and if yes try to use it 
as is.

--
Cheers
Swedebugia