Read file and convert content into a list of strings.
DESCRIPTION
When given a filename, the function strings_of_file attempts to open the file
for input, and if this is successful reads and closes it, returning a list of
strings corresponding to the lines in the file.
FAILURE CONDITIONS
Fails if the file cannot be opened (e.g. it does not exist, or the permissions
are wrong).
EXAMPLE
If the file /tmp/greeting contains the text
Hello world
Goodbye world
then
# strings_of_file "/tmp/greeting";;
val it : string list = ["Hello world"; "Goodbye world"]