proposal: with-file-writeable

[Gábor Boskovits]

the make-file-writeable function seems a bit too imperative to me, it would
look better if we could have a with-file-writeable function, so that we can
constrain the size effect, and more. Moreover if a file is read-only to
start with, it might be a good idea to keep it that way anyways. WDYT?