`install-file' should print out verbose logs

[Arun Isaac]

`install-file' in (guix build utils) should print out verbose logs just
as `copy-recursively' does. Something like:

(format log-port "~a -> ~a~%" source destination)

It's nice to have flowing verbose output when building a package. I
believe it is "more reassuring" than waiting at a blank screen with a
long pause when files are being installed.

Putting the logging in the `install-file' procedure itself, will save us
the trouble of having to implement logging manually everywhere
`install-file' is used.

In an earlier conversation at
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=26941#26 , Ludo said that
he's neither for or against this proposal, and that we can implement it
if there is enough support from the community.

So, please share your thoughts.

Thank you! :-)