`char` type is not compatible with all boards

[tomcombriat]

Trying to get the AudioDelayFeedbackX2 example to work correctly on the Pico (RP2040) I encountered a weird behavior: char type on the Pico (and maybe some other boards) is unsigned by default. Here is a small example that exemplifies that and all the potential side effects this could have:

char a = -1;

void setup() {
  Serial.begin(115200);
}

void loop() {
  Serial.println(sizeof(a));
  Serial.println(a - 1);
  delay(1000);
}

Output:

1
254

Note that it does work with the expected behavior on the UNO or by defining signed char a=-1.

---

For mozzi, the expected type in most cases is int8_t, which is the same for everyone, hence I think it will be beneficial to have a critical look at every places where char is used and change it when necessary (probably everywhere as there are no example that I can think of which actually uses characters).
This goes along the line that I remember @tfry-git saying about defining more closely the types we are working with.