`Stream::find` does incorrect comparison

This code hangs forever, even when `Serial1` repeatedly receives `0xCA`:

```cpp
while (!Serial1.find(0xCA)) ;
```

The reason is in `Stream::findMulti`:

https://github.com/arduino/ArduinoCore-avr/blob/86df345b3cf46754a5db38fb983ec2808ce31303/cores/arduino/Stream.cpp#L270

Consider a comparison where:

- `c` is `int` with an unsigned value `0xCA`.
- `t->str` is `const char *` with a signed value `-54` decimal, which as an unsigned value is `0xCA`.

In the comparison, the char is sign extended to an `int`, resulting in `0xFFFFFFCA` giving:

```cpp
if (0xCA == 0xFFFFFFCA) {
```

This is never `true` and not the intention. There appear to be other comparisons in `Stream.cpp` with this problem.

### Additional context

Tested on Arduino Mega 2560 R3.

#### Related

- https://github.com/arduino/ArduinoCore-API/pull/274
- https://github.com/arduino/ArduinoCore-avr/issues/249

Start in cores/arduino/Stream.cpp at Stream::findMulti and inspect the comparison at the linked line, then review the other Stream.cpp comparisons mentioned in the issue. Reproduce the Serial1.find(0xCA) example on the Mega 2560 setup; done means the matching byte is recognized and the loop no longer hangs.

Written by the indexing model from the issue text.