A library of functions implementing a useful file system.

Necessary functions (you decide the exact details of the parameters,
but it must be easy and convenient to use the functions in a normal
program).

Top priority:

+ format

    Initialise a new disc (or reinitialise an existing one), preparing
    it for its first use.

+ open

    Given a file name, and an indication of whether the file is to be
    opened for reading or writing. Check that the file is accessible
    and prepare it for access, creating a suitably sized buffer.

+ close

    Exactly what close normally does to a file.

+ readchar

    Fail if the file is not open for reading. If the buffer has no unread
    content left, refill it from the file first. Return the next unread
    character from the buffer. When at the end of the file, return some
    sensible value to signify that fact.

+ writechar

    Fail if the file is not open for writing. If the buffer is full, empty
    it into the file first. Put the supplied character into the buffer.

+ length

    Return the length (in bytes) of the file.

After that:

+ add a function for deleting a file

+ add support for listing a directory

+ add support for creating and using sub-directories

+ with just readchar and writechar, you can read and write any kind of data
  to a file, but it isn't very convenient. Provide some convenient user-level
  access functions, such as read_number, read_line, write_number, write_string.