STDERR
* stdout is for normal output.
* stderr is for diagnostic / exception / error output.
* None of this is enforced by the system.
  - For HW06/HW07/HW08, we have a specification.
  - log_▒▒▒▒(…) (functions HW06) go to stdout because we might want to compare that output
    to expected.txt as part of normal testing.
  - mu_run(…) (key function from HW07) goes to stderr because comparing
    ".....  test passed" with expected would make no sense.
* Both go to the terminal by default.
* They can be redirected separately.
  - ./test_count_words  >  test_count_words.txt   # redirects stdout  ← REMEMBER THIS
  - ./test_count_words 2>  test_count_words.txt   # redirects stderr  ← OPTIONAL INFO
  - ./test_count_words &>  test_count_words.txt   # redirects both    ← OPTIONAL INFO
* For HW07, we will try to be lenient.
* For HW08, please make sure to follow this.
* If you had a __mu_print_color(…), you may need a separate one, like
  __mu_print_color_stderr(…) in your miniunit.h.  You also need to adjust
  the isatty(…) to use isatty(STDERR_FILENO).
* Makefile messages that say test passed or failed should be on stderr.
* STUPID WAY TO BYPASS SELF-TESTING REQUIREMENT FOR HOMEWORK.  DO NOT DO THIS ON NORMAL
  ASSIGNMENTS
  $ ./test_count_words  >  expected.txt


MALLOC(…)
* malloc(…) allocates a buffer in memory (reserves space).
* It's sole parameter is the number of bytes.
* malloc(…) returns the address of the new buffer.
* For every call to malloc(…) there must be exactly one call to free(…).
* Call free(…) on the same address that malloc(…) returned.

    char* divider = malloc( (num_repetitions + 2) * sizeof(*divider) );
    //                      └────────┬──────────┘   └───────┬──────┘
    //                         number of chars    number of bytes to store one char