Given a filename fn and a string s, the call file_of_string fn s attempts
to open the file fn for writing and writes the string s to it before
closing. If the file exists, it will be overwritten, and otherwise a new file
will be created.
FAILURE CONDITIONS
Fails if the file cannot be opened for writing.
EXAMPLE
The call
# file_of_string "/tmp/hello" "Hello world\nGoodbye world";;
val it : unit = ()
will result in a file /tmp/hello containing the text: