pw_stream: Use c++20-friendly pointer operations on spans
pw_stream: Use c++20-friendly pointer operations on spans

With C++20 enabled pigweed uses std::span from the standard library.
It doesn't specify they iterator type and it is not always possible to
cast it to the pointer directly. We need to use .data() instead.

Change-Id: I4bcb6a924c0f1d4a22ef41286921bc006bc91f60
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/98905
Pigweed-Auto-Submit: Dennis Kormalev <6222e5e705620133a24041cba003e993480020a9@google.com>
Reviewed-by: Armando Montanez <97ef6fdb231c129ac8827d9f0c4106bfbf57f74f@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed