Migrate Stream search internals from char to uint8_t

[asumo-1xts]

Overview

This PR is regarding issue #119.

With the current code, Stream::find() cannot find values above 0x7f because it treats the target string as char, meaning that many UTF-8 encoded characters cannot be matched either.
This PR fixes the issue by changing the target string type from char to uint8_t.

Verification on an actual device

Find single-byte number above 0x7f

When 0x80 is sent via the serial monitor, the sketch below returns "Not found.” with current code while it returns “Found!” this PR's fixed code.

#include <Arduino.h>

const uint8_t TARGET_NUM = 0x80; // above 0x7f

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

void loop() {
  if (Serial.available()) {
    if (Serial.find(TARGET_NUM)) {
      Serial.println("Found!");
    } else {
      Serial.println("Not found.");
    }
  }
}

Find multi-byte UTF-8 character

With the current code, the sketch below fails to build with the error no instance of overloaded function "Serial_::find" matches the argument list. With the fixed code, it builds and works as expected.

#include <Arduino.h>

const uint8_t TARGET_STR[] = "やあ😀";
// E3 82 84 + E3 81 82 + F0 9F 98 80

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

void loop() {
  if (Serial.available()) {
    if (Serial.find(TARGET_STR)) {
      Serial.println("Found!");
    } else {
      Serial.println("Not found.");
    }
  }
}

[CLAassistant]

All committers have signed the CLA.

[asumo-1xts]

I just realized that this PR might also fix these issues:
arduino/ArduinoCore-avr#249
arduino/ArduinoCore-avr#541

[asumo-1xts]

Sorry if my repeated notifications have been noisy.
I have revised my original comment and have nothing further to add at this point.
If there is anything else I should address, please let me know.