`random` and `rand` are inconsistent

[ZERICO2005]

We provide two random number generation functions:

uint32_t random(void); // [0, 2^32)
int rand(void); // [0, 2^23)

Currently rand() wraps random(). To do this, it just needs to clear bit 23 to make sure the result is [0, RAND_MAX] or [0, 2^23). I believe the intended implementation is return (random() & 0x7FFFFF), however, since the sbc instruction is used in a place where carry is unknown, the result may or may not be off by one (Although it will still be [0, RAND_MAX]).

_rand:
    ; the last operation performed by _random is add hl, de \ adc a, c \ ld e, a \ ret
    ; therefore the state of carry is indeterminate
    call    _random
    ld    de, 8388608
    sbc    hl, de ; carry is unknown here
    ret    p
    add    hl, de
    ret

We can fix it so rand() always does return (random() & 0x7FFFFF) by doing this:

_rand:
    call    _random
    ld    de, 8388608
    add hl, de
    ret    c
    add    hl, de
    ret

Pros:

• rand() and random() produce consistent results
• 1 byte smaller

Cons:

• rand() will output different results when recompiling a program. More specifically, 50% of the results from rand() will be one higher than they were before.

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Here is how the test cases (#760) differ for rand with this change. rand_X calls srand every iteration, and since the initial state is 1,2,3,4,5,... carry will almost never be set. But rand_0 does call rand several times, allowing the state to become more randomized and start emitting carrys