Calling parse_element(…) on "1" will result in an Element object containing the int 1.
Equivalent to

Element element = { .as_int = 1, .type = ELEMENT_INT }; 
... but returned by address.

This is imilar to how parse_int(…) initialized an int by address and
parse_string(…) initialized a char* by address.

parse_element(…):
If the input string (at *a_pos) start with a digit, then you call parse_int(…)
                                               '"', then you call parse_string(…)
                                               '[', then you call parse_list(…)
Your parse_element(…) should not look at individual characters any more than eating
whitespace and deciding which function to call.  It should delegate all of the actual
parsing to parse_int(…), parse_string(…), and parse_list(…)


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parse_list(…)

parse_list(…) with "[1,2]"
1 Eat the '['.  (If it wasn't there, you return false.)
2 Call parse_element(…)
  - WARNING:  Do not try to parse the value (int, string, etc.) or even call parse_int(…)
    and such from within parse_list(…).  parse_list(…) must not need to know the types
    that an Element object can store.
3 Eat either a ',' (if list is continuing) or a ']' if that was the end of the list.
4 If list is continuing, then go back to step 2.

If you pass "[]" to parse_list(…), then the list is empty, so you set *a_head to NULL.
- same goes for "[  ]"

HOW TO START HW13 (suggestion)
- parse_element(…)
- parse_json(…)
- parse_list(…)