gh-107758: Improvements to lltrace feature (#107757)

- The `dump_stack()` method could call a `__repr__` method implemented in Python,
  causing (infinite) recursion.
  I rewrote it to only print out the values for some fundamental types (`int`, `str`, etc.);
  for everything else it just prints `<type_name @ 0xdeadbeef>`.

- The lltrace-like feature for uops wrote to `stderr`, while the one in `ceval.c` writes to `stdout`;
  I changed the uops to write to stdout as well.