secrm: Example of writing "secure" file removal in Haskell rather than C.

An example of code to "securely" remove a file from media by over-writing
the file several times with random data, and then erasing it.

Note that this is just an example intended to show that one can write
system utilities in Haskell instead of, say, C. It does not, for example,
guarantee that the underlying OS/FS will be writing the random data over
exactly the same blocks. (In hindsight, perhaps making this not be the best
example. Living and learning, here.)